Lets create a python 3 repl, and call it Hello World. Now you have a blank file called main.py. Now let us write our first line of code:
helloworld.py
print('Hello world!')
Brian Kernighan actually wrote the first “Hello, World!” program as part of the documentation for the BCPL programming language developed by Martin Richards.
Now, press the run button, which obviously runs the code. If you are not using replit, this will not work. You should research how to run a file with your text editor.
If you look to your left at the
console where hello world was just printed, you can see a >, >>>, or $ depending on what you are using.
After the prompt, try typing a line of code.
Python 3.6.1 (default, Jun 21 2017, 18:48:35)[GCC 4.9.2] on linuxType "help", "copyright", "credits" or "license" for more information.> print('Testing command line')Testing command line> print('Are you sure this works?')Are you sure this works?>
The command line allows you to execute single lines of code at a time. It is often used when trying out a new function or method in the language.
Another cool thing that you can
generally do with all languages, are comments. In python, a comment starts with a
#. The computer ignores all text
starting after the #.
shortcom.py
# Write some comments!
If you have a huge comment, do
not comment all the 350 lines, just
put ''' before it, and ''' at the end. Technically, this is
not a comment but a string, but the computer still ignores it, so we will use
it.
longcom.py
'''Dear PYer,I am confused about how you said you could use triple quotes to makeSUPERLONGCOMMENTS!I am wondering if this is true,and if so,I am wondering if this is correct.Could you help me with this?Thanks,Random guy who used your tutorial.'''print('Testing')
Unlike many other languages,
there is no var, let, or const to declare a variable in python.
You simply go name =
'value'.
vars1.py
x = 5y = 7z = x*y # 35print(z) # => 35
Remember, there is a difference
between integers and strings. Remember: String = "". To convert between these two,
you can put an int in a str() function, and a string in a
int() function. There is also a less
used one, called a float. Mainly, these are integers with decimals. Change them using
the float() command.
vars2.py
x = 5x = str(x)b = '5'b = int(b)print('x = ', x, '; b = ', str(b), ';') # => x = 5; b = 5;
Instead of using the , in the print function, you can
put a + to combine the variables and
string.
There are many operators in python:
+
-
/
* These operators are the
same in most languages, and allow for addition, subtraction, division, and
multiplicaiton. Now, we can look at a few more complicated
ones:
%
//
**
+=
-=
/=
*= Research these if you want
to find out more…
simpleops.py
x = 4a = x + 1a = x - 1a = x * 2a = x / 2
You should already know everything shown above, as it is similar to other languages. If you continue down, you will see more complicated ones.
complexop.py
a += 1a -= 1a *= 2a /= 2
The ones above are to edit the
current value of the variable.
Sorry to JS users, as there is no i++; or
anything.
Fun Fact: The python language was named after Monty Python.
If you really want to know about the others, view Py Operators
Like the title?
Anyways, a ' and a " both indicate a string, but
do not combine
them!
quotes.py
x = 'hello' # Goodx = "hello" # Goodx = "hello' # ERRORRR!!!
slicing.py
You can look at only certain
parts of the string by slicing it, using [num:num].
The first number stands for how far in you go from the front, and the second stands
for how far in you go from the back.
x = 'Hello everybody!'x[1] # 'e'x[-1] # '!'x[5] # ' 'x[1:] # 'ello everybody!'x[:-1] # 'Hello everybod'x[2:-3] # 'llo everyb'
Here is a list of functions/methods we will go over:
.strip()
len()
.lower()
.upper()
.replace()
.split()
I will make you try these out yourself. See if you can figure out how they work.
strings.py
x = " Testing, testing, testing, testing "print(x.strip())print(len(x))print(x.lower())print(x.upper())print(x.replace('test', 'runn'))print(x.split(','))
Good luck, see you when you come back!
Input is a function that gathers input entered from the user in the command line. It takes one optional parameter, which is the users prompt.
inp.py
print('Type something: ')x = input()print('Here is what you said: ', x)
If you wanted to make it smaller, and look neater to the user, you could do…
inp2.py
print('Here is what you said: ', input('Type something: '))
Running: inp.py
Type something:Hello WorldHere is what you said: Hello World
inp2.py
Type something: Hello WorldHere is what you said: Hello World
Python has created a lot of
functions that are located in other .py files. You need to import these modules to gain access to the,, You may
wonder why python did this. The purpose of separate modules is to make python faster.
Instead of storing millions and millions of functions, , it only needs a few basic
ones. To import a module, you must write input
<modulename>. Do not add the .py extension to
the file name. In this example , we will be using a python created module named
random.
module.py
import random
Now, I have access to all
functions in the random.py file. To access a specific function in the module, you
would do <module>.<function>. For
example:
module2.py
import randomprint(random.randint(3,5)) # Prints a random number between 3 and 5
Pro Tip: Do
from random import randintto not have to dorandom.randint(), justrandint()To import all functions from a module, you could dofrom random import *
Loops allow you to repeat code over and over again. This is useful if you want to print Hi with a delay of one second 100 times.
for Loop
The for loop goes through a list of variables, making a seperate variable equal one of the list every time. Let’s say we wanted to create the example above.
loop.py
from time import sleepfor i in range(100):print('Hello')sleep(.3)
This will print Hello with a .3 second delay 100 times. This is just one way to use it, but it is usually used like this:
loop2.py
import timefor number in range(100):print(number)time.sleep(.1)
while Loop
The while loop runs the code
while something stays true. You would put while
<expression>. Every time the loop runs, it
evaluates if the expression is True. It it is, it runs the code, if not it continues
outside of the loop. For example:
while.py
while True: # Runs foreverprint('Hello World!')
Or you could do:
while2.py
import randomposition = '<placeholder>'while position != 1: # will run at least onceposition = random.randint(1, 10)print(position)
The if statement allows you to check if something is True. If so, it runs the code, if not, it continues on. It is kind of like a while loop, but it executes only once. An if statement is written:
if.py
import randomnum = random.randint(1, 10)if num == 3:print('num is 3. Hooray!!!')if num > 5:print('Num is greater than 5')if num == 12:print('Num is 12, which means that there is a problem with the python language, see if you can figure it out. Extra credit if you can figure it out!')
Now, you may think that it would
be better if you could make it print only one message. Not as many that are True. You
can do that with an elif statement:
elif.py
import randomnum = random.randint(1, 10)if num == 3:print('Num is three, this is the only msg you will see.')elif num > 2:print('Num is not three, but is greater than 1')
Now, you may wonder how to run
code if none work. Well, there is a simple statement called else:
else.py
import randomnum = random.randint(1, 10)if num == 3:print('Num is three, this is the only msg you will see.')elif num > 2:print('Num is not three, but is greater than 1')else:print('No category')
So far, you have only seen how to use functions other people have made. Let use the example that you want to print the a random number between 1 and 9, and print different text every time. It is quite tiring to type:
Characters: 389
nofunc.py
import randomprint(random.randint(1, 9))print('Wow that was interesting.')print(random.randint(1, 9))print('Look at the number above ^')print(random.randint(1, 9))print('All of these have been interesting numbers.')print(random.randint(1, 9))print("these random.randint's are getting annoying to type")print(random.randint(1, 9))print('Hi')print(random.randint(1, 9))print('j')
Now with functions, you can seriously lower the amount of characters:
Characters: 254
functions.py
import randomdef r(t):print(random.randint(1, 9))print(t)r('Wow that was interesting.')r('Look at the number above ^')r('All of these have been interesting numbers.')r("these random.randint's are getting annoying to type")r('Hi')r('j')
This video should help you get up and running with Python 3
Installing Python is really a cakewalk. Search for “Python download” on www.google.com. Download the installable and install it.
A quick word of caution on Windows
Make sure that you have the check-box “Add Python 3.6 to PATH”, checked.
Once you have installed Python, you can launch the Python Shell.
Windows - Launch cmd prompt by typing in ‘cmd’ command.
Mac or Linux - Launch up terminal.
Command to launch Python 3 is different in Mac.
In Mac, type in
python3
In other operating systems,
including windows, type python
You can type code in python shell and code as well!
You can use print(5*4), and it shows 20.
You can execute the code, and the shell would immediately give you output.
Using the the Python Shell is an awesome way to learn Python.
Most programmers find programming a lot of fun, and besides, it also gets their work done.
Programming mainly involves problem solving, where one makes use of a computer to solve a real world problem.
During our journey here, we will approach programming in a very different way. We will not only introduce you to the Python language, but also help you pick up essential problem solving skills.
As a programmer, you need to be able to look at a problem, and identify the important programming concepts relevant to solving it. Finally, you need to be able to use the language features and syntax, to express your solution on the computer. While all this looks complex, we want to make it easy for you. Together, we will tackle a variety of programming challenges, using these same steps. We will start with simple challenges (such as a Multiplication Table), and gradually increase the difficulty level over the duration of this book.
Learning to program is a lot like learning to ride a bicycle. The first few steps are the most challenging ones.
Once you get over these initial steps, your experience will become more and more enjoyable.
Are you ready for your first programming challenge? Let’s get going now! We wish you all the best.
Summary
In this step, we:
Were introduced to the concept of problem solving
Understood how good programmers approach problem solving
Our first programming challenge aims to do, what every kid does in math class: read out a multiplication table. We now want to give this task to the computer. Here is the statement of our problem:
The Print Multiplication Table Challenge (PMT-Challenge)
Compute the multiplication
table for 5, with entries from
1 to 10.
Display this table.
The display needs to be:
5 * 1 = 5
5 * 2 = 10
5 * 3 = 15
5 * 4 = 20
5 * 5 = 25
5 * 6 = 30
5 * 7 = 35
5 * 8 = 40
5 * 9 = 45
5 * 10 = 50
This is the challenge. For convenience, let’s give it a label, say PMT-Challenge. What would be the important concepts we need to learn, to solve this challenge? The following list of concepts would be a good starting point:
Statements
Expressions
Variables
Literals
Conditionals
Loops
Methods
In the rest of this chapter, we will introduce these concepts to you, one-by-one. We will also show you how learning each concept, takes us closer to a solution to PMT-Challenge.
Summary
In this step, we:
Stated our first programming challenge
Identified what programming concepts we need to learn, to solve this challenge
Typically when we do programming, we have problems. Solving the problem typically need a step-by -step approach. Common sense tells us that to solve a complex problem, we break it into smaller parts, and solve each part one by one. Here is how any good programmer worth her salt, would solve a problem:
Simplify the problem, by breaking it into sub-problems
Solve the sub-problems in stages (in some order), using the language
Combine these solutions to get a final solution
The PMT-Challenge is no different! Now how do we
break it down, and where do we really start? Once again, your common sense will reveal
a solution. As a first step, we could get the computer to calculate say, 5 * 3. The second thing we can do, is
to try and print the calculated value, in a manner similar to 5 * 3 = 15. Then, we could repeat what we
just did, to print out all the entries of the 5 multiplication table. Let’s put
it down a little more formally:
Here is how our draft steps look like
Calculate 5 * 3 and print result as
15
Print 5 * 3 = 15 (15 is result of previous
calculation)
Do this ten times, once for
each table entry (going from 1 to 10)
Let’s start with that kind of a game plan, and see where it takes us.
Summary
In this step, we:
Learned that breaking down a problem into sub-problems is a great help
Found a way to break down the PMT-Challenge problem
Let’s focus on solving the first
sub-problem of PMT-Challenge, the numeric computation. We
want the computer to calculate 5 * 5 for example, and print
25 for us. How do we get it to do
that? That’s what we would be looking at in this step.
Snippet-01: Introducing Operators
Launch up Python shell. We want
to calculate 5 *
5. How do
we do that?
Using our knowledge of school
math, let’s try 5 X 5.
>>> 5 X 5File "< stdin >", line 15 X 5^SyntaxError: invalid syntax
The Python Shell hits back at us, saying “invalid syntax”. This is how Python complains, when it doesn’t fully understand the code you type in. Here, it says our code has a “SyntaxError”.
The reason why it complains, is
because ‘X’ is not a valid operator in Python.
The way you can do multiplication
is by using the ‘*’ operator .
“5 into
5” is
achieved by the code 5 * 5, and you can see the result
25 being printed. Similarly,
5 *
6 gives us
30.
>>> 5 * 630
There are a wide range of other operators in Python:
5 + 6 gives a result of
11.
5 - 6 leads to -1.
>>> 5 + 611>>> 5 - 6-1
10 / 2, gives an output of 5.0 . There is one interesting
operator, **. Let’s try 10 ** 3. We ran this code, and the
result we get is 1000. Yes you guessed right, the
operator performs “to the power of”. “10 to the power of 3” is 10 * 10 * 10, or 1000.
>>> 10 / 25.0>>> 10 ** 31000
Another interesting operator is
%, called “modulo”, which computes the remainder
on integer division. If we do 10 % 3, what is the remainder when
10 is divided by 3? 3 * 3 is 9, and 10 - 9 is 1, which is what % returns in this
case.
Let’s look at some terminology:
Whatever pieces of code we
gave Python shell to run, are called expressions. So, 5 * 5, 5 * 6 and 5 - 6 are all expressions. An expression is
composed of operators and operands.
In the expression
5 *
6,
the two values 5 and 6 are called operands, and
the * operator operates on
them.
The values 5 and 6 are literals, because those are
constants which cannot be changed.
The cool thing about Python, is
that you can even have expressions with multiple operators. Therefore, you can form an
expression with 5 + 5 + 5, which evaluates to 15. This is an expression which has
three operands, and two + operators. You can even have
expressions with different types of operators, such as in 5 + 5 * 5.
>>> 5 + 5 + 515>>> 5 + 5 * 530
Try and play around with the expressions, and understand the output which results.
Summary
In this step, we:
Learned how to give code input to the Python Shell
Understood that Python has a predefined set of operators
Used a few types of basic operators and their operands, to form expressions
At this stage, your smile tells us that you enjoy evaluating Python expressions. What if we tickle your mind a bit, to make sure it hasn’t fallen asleep? Here is your first programming exercise.
Exercises
Write an expression to calculate the number of minutes in a day.
Write an expression to calculate the number of seconds in a day.
Note
You need to solve these problems by yourself. If you are able to work them out, that’s fantastic! But if not, that’s part of the learning process.
Solutions
Solution 1
>>> 24 * 601440
We wanted to calculate the number of minutes in a day. How do we do that? Think about this…
How many number of hours
are there in a day? 24.
And how many minutes does
each hour have? It’s 60.
So if you want to find out
the number of minutes in a day, it’s 24 * 60, which is 1440.
Solution 2
>>> 24 * 60 * 6086400
How many seconds are there in a day?
Let’s start with the number
of hours, 24.
The number of minutes in an
hour is 60, and
The number of seconds in a
minute is 60 as
well.
So it’s 24 * 60 * 60, or 86400.
Summary
In this step, we:
Solved a Programming Exercise involving common scenarios, using Python code involving:
Expressions
Operators
Literals
Let’s look at a few puzzles related to expressions, in this step. Before that, let’s revise some of the terminology we had learned earlier.
5 + 6 + 10 is an example of an expression.
In this expression, 5, 6 and 10 are operands. The + here is the operator. You can
have multiple operators in an expression. We also did mention that the operands,
namely 10, 6 and 5, are literals. Their values
will not change.
Here are a few puzzles coming up, to explore aspects of expressions.
Snippet-01: Puzzles On Expressions
Think about what would happen
when you do something of this kind: 5 $ 2. You’re right, it would throw a
SyntaxError. When Python does not understand
the code you type in, it reports an error. Here, the expression we’re typing is
5 $
2, which
does not make sense to Python, hence the SyntaxError.
>>> 5 $ 2File "< stdin >", line 15 $ 2^SyntaxError: invalid syntax>>> 5$2File "< stdin >", line 15 $ 2^SyntaxError: invalid syntax
Let’s say we type in 5+6+10, without any spaces between the
operands, and the operators. What do you think will happen? Surprisingly, the Python
Shell does calculate the value!
>>> 5+6+1021
In an expression, using spaces
makes it easier for you to read it, but it’s not mandatory. 5 + 6 + 10 is easier to read than
5+6+10, but does not make any
difference to the Python compiler.
The next puzzle tries to evaluate
5 /
2, which is
“5 divided by 2”. What would be the output?
2.5.
>>> 5/22.5
If you’re coming from other
programming languages like Java or C, this might be a surprising result. If you try
this in Java for instance, you would get 2 as the output. Note that even
though both operands are integers, the result of the / operation is a floating point
value, 2.5 . Python does what is expected
by a programmer!
The puzzle after that tries to
play with 5 + 5 *
6. What
would be the result of this expression? Will it be 5 + 5 or 10, then 10 * 6, which is 60? Or, will it be 5 plus 5 * 6, which is 5 + 30, that’s 35?
>>> 5 + 5 * 635
The correct result is
35.
Python decides this is based on the precedence of operators.
Operators in Python are divided into two sets as follows:
**, *, / and % have higher precedence, or
priority.
+ and - have a lower
precedence.
Sub-expressions involving
operators from {*, /, %, **} are evaluated before those
involving operators from {+, -}
Let’s try another small puzzle on
precedence, with 5 - 2 * 2. What would be the result of
this? Will it be 6, or 1? It’s 1, because * has a higher precedence than
-. Thus 2 * 2 is 4, and 5 - 4 gives us 1.
>>> 5 - 2 * 21
Let’s say we want to execute
5 -
2, to give
an output of 2. How do we change the operator
precedence?
You cannot really change the precedence, but you can add parentheses to group sub-expressions differently.
>>> (5 - 2) * 26>>> 5 - ( 2 * 2 )1
Parentheses have the highest
precedence in Python, and can be used to override operator precedence. (5 - 2) gets calculated first, and the
final result of the expression is 6.
A positive thing about using
parentheses is, that it makes expressions more readable. So even in situations such as
5 - 2 *
2, where we
know the result according to precedence, adding parentheses is
good.
Summary
In this step, we went about solving a few puzzles about expressions, touching concepts such as:
SyntaxError for incorrect
operators
White-space in expressions
Floating Point division by default
Operator Precedence
Using parentheses
In the previous step, we learned how to use expressions to compute values. In this step, let’s see how we can actually print multiplication table entries, that are readable by the user.
Snippet-01: Printing Text
How do we go about printing a
complete multiplication table entry? We want to print text such as 5 * 6 = 30 . But trying to do so, as we
know it, gives us a SyntaxError. Clearly, there is a different
way to print text, as compared to an expression.
>>> 5 * 6 = 30File "<stdin>", line 1SyntaxError: can't assign to operator
Let’s first try to print a simple
piece of text, Hello. Typing in this piece of code
directly on Python Shell also gives us an error.
>>> HelloTraceback (most recent call last):File "<stdin>", line 1, in <module>NameError: name 'Hello' is not defined
Only expressions work that way,
and Hello is not really an
expression.
"Hello" is typically called a
string, and represents the text of
letters 'H', 'e', 'l', 'l', 'o'. "Hello" is hence different from the
number 5.
There are a number of in-built
functions in Python to help print strings. One of these is the print() function. Can you just say
print
Hello?
>>> print HelloFile "<stdin>", line 1print Hello^SyntaxError: Missing parentheses in call to 'print'. Did you mean print(Hello)?
The Python compiler gives you an error, that says “missing parentheses”.
Will print(Hello) work?
>>> print (Hello)Traceback (most recent call last):File "<stdin>", line 1, in <module>NameError: name 'Hello' is not defined
Nope! Again, this one failed
because you need to indicate that "Hello" is a
string.
How do I indicate that
"Hello" is a string? By putting it
within double quotes.
Let’s try print
("Hello")
>>> print ("Hello")Hello>>> print("Hello")Hello
print("Hello") finally results in "Hello" being printed out. To be able to
print "Hello", the things we need to do
are:
Typing the method name print ,
open parentheses ( ,
Followed by a double quote " ,
The text Hello,
and another double quote " ,
finished off with a closed parentheses ).
What we have written here is
called a statement, a simple piece of code to
execute. As part of this statement, we are calling a function, named print().
What exactly are we trying to print?
The text "Hello", which is called a parameter or argument, to print().
Now let’s get back to what we
wanted to do, which is to print 5 * 6 = 30. The most basic version would be
something of this kind, print("5 * 6 =
30").
Here, we are passing the entire value in the form of a string.
>>> print("5 * 6 = 30")5 * 6 = 30
This prints the text on the
console, as-is. The thing you need to understand here is, we aren’t really calculating
30 using the formula 5 * 6, but directly putting text
30 in here. That’s called
hard-coding.
In a later step, we will look at how to actually calculate the value and pass it in.
Summary
In this step, we:
Understood that displaying text on the console is not the same as printing an expression value
Learned about the
print() function, that is used to
print text in Python.
Found a way to print the
text "5 * 6 =
30" on the console, by
hard-coding values in a string
In the previous step, we learned
how to print 5 * 6 =
30. It was
not a perfect solution, because we hard-coded everything. we used an in-built function
named print(), passed a string to it, and
invoked the method.
In this step, let’s look at a number of puzzles related to in-built methods, their parameters, and strings in general.
For example, let’s do
print("5 *
6"),
as in the previous step. What does this code result in?
>>> print("5*6")5*6>>> print('5*6')5*6
It just prints the string
"5 *
6".
Let’s say we try the code
print(5 *
6),
>>> print(5*6)30
Without the double quotes,
5 *
6 is an
expression. What will be the output? 30.
If you call print() with an expression argument, it
prints the value of the expression. However, when we pass something within double
quotes, it becomes a piece of text, printed as-is.
An interesting thing to note is,
that in Python you can use either double-quotes (" and "), or single-quotes (' and ') with text
values.
Let’s look at a few other in-built methods within Python.
Consider abs() (which stands for absolute
value), a method that accepts a numeric value. You can use abs(10.5), passing 10.5 as a value to it, and it prints
the absolute value of 10.
>>> abs 10.5File "<stdin>", line 1abs 10.5^SyntaxError: invalid syntax>>> abs(10.5)10.5
If you pass in a string value,
will it work? It complains, “abs() function will not work with a
string, it only works with numeric values”.
>>> abs("10.5")Traceback (most recent call last):File "<stdin>", line 1, in <module>TypeError: bad operand type for abs(): 'str'
Let’s say you want to use a
function that computes “to the power of”, for instance “2 to the power of 5”. In Python, there’s an in-built
function named pow(), which does just what we need.
To pow(), you can pass two parameters and
calculate the result. How do you do that?
Will this work: pow 2 5? No, not at all. This code does
not work as well: pow(2 5). pow(2, 5) is the correct
syntax.
>>> pow 2 5File "<stdin>", line 1pow 2 5^SyntaxError: invalid syntax>>> pow(2 5)File "<stdin>", line 1pow(2 5)^SyntaxError: invalid syntax>>> pow(2, 5)32
You’ll see that 32 is printed.
Let’s see another example,
“10 to the power of 3”. pow(10,3) is the alternative to saying
10 **
3. This
gives us 1000, similar to how pow() would.
>>> pow(10, 3)1000>>> 10 ** 31000
max() returns maximum in a set of
numbers.min() function returns the minimum
value.
>>> max(34, 45, 67)67>>> min(34, 45, 67)34
These are some of the in-built functions in Python, and we saw how to call the in-built functions by passing in a varied number of parameters.
Python is case sensitive. So
let’s say I want of calculate pow(2,5). So this would give me
32. Now, what if I say capital
'P' instead of small 'p' here? Pow(2,5) would lead to an
error.
>>> pow(2,5)32>>> Pow(2,5)Traceback (most recent call last):File "<stdin>", line 1, in <module>NameError: name 'Pow' is not defined
The only things not
case-sensitive in Python, are string values. Earlier we saw that the code print("Hello") displays the text "Hello". Inside a string, the text can
be in any case. Hence, print("hello") displays "hello" ,with a small 'h'.
>>> print("Hello")Hello>>> print("hello")hello>>> print("hellO")hellO>>> print ( "hellO" )hellO
However inside your code, you need to be very particular about the case of function names, class names, variable names, and the like.
In your code, whitespace does not really matter. You can add space here and here, and you would still get the same output. However, in case of strings, whitespace does matter.
If we say print("hellO
World"), it would print "hellO
World", with a space in between. And if
you do print("hellO
World") with three spaces, it would
print the same. In expressions, white-space does not affect the
output.
>>> print ( "hellO World" )hellO World>>> print ( "hellO World" )hellO World
The last thing we want to look
at, is an escape sequence. Let’s say you want to print a
double quote, ", in the code. If we were to do
this: print("Hello""), what would happen? The compiler
says error!
>>> print("Hello"")File "<stdin>", line 1print("Hello"")^SyntaxError: EOL while scanning string literal
If you want to print a
" inside a string, use an escape
sequence. In Python, the symbol '\' is used as an escape character. On using '\' adjacent to the ", it prints Hello" (notice the trailing
"). We have used the '\' to escape the ", by forming an escape sequence \".
>>> print("Hello\"")Hello">>>
The other reason why you would
want to use a '\' is to print a <NEWLINE>. If you want to print
"Hello
World", but with "Hello" on one line and "World" on the next, '\n' is the escape sequence to
use.
>>> print("Hello\nWorld")HelloWorld
The other important escape
sequence is '\t', which prints a <TAB> in the output. When you do
print("Hello\tWorld"), you can see the tab-space
between "Hello" and "World".
>>> print("Hello\tWorld")Hello World
Another useful escape sequence is
\\ . If you want to print a
\ , then use the sequence
\\ . You would see that it prints
Hello\World . Think about what would happen
if we put six \ . Yes you’re right! It would
print this string: "\\\" .
>>> print("Hello\\World")Hello\World>>> print("Hello\\\\\\World")Hello\\\World
One of the things with Python is,
it does not matter whether you use double quotes or single quotes to enclose strings.
There are some interesting, and useful ways of using a combination of both, within the
same string. Have a look at this call: print("Hello'World"), and notice the output we get.
In a similar way, the following code will be accepted and run by the Python system:
print('Hello"World').
>>> print('Hello"')Hello">>> print("Hello'World")Hello'World>>> print("Hello\"World")Hello"World>>> print("Hello\"World")Hello"World
The above two examples can be used as a tip by newbie programmers when they form string literals, and want to use them in their code:
If the string literal contains one or more single quotes, then you can use double quotes to enclose it.
However if the string contains one or more double quotes, then prefer to use single quotes to enclose it.
Summary
In this step, we:
Explored a number of puzzles related to code involving:
Built-in functions for numeric calculations
The print() function to display
expressions and strings
Covered the following aspects of the above utilities:
Case-sensitive aspects of names and strings
The role played by whitespace
The escape character, and common escape sequences
In the previous step, we learned
how to print a hard-coded string, such as "5 * 6 =
30".
In this step, let’s try to
replace the hard-coded 30 with a computed
value.
Let’s start with a simple scenario. Let’s say we want to place that calculated value within a string, and display it. How do we do that?
Snippet-01: print() Formatted Output
format() method can be used to print
formatted text.
Let’s see an example:
>>> print("VALUE".format(5*2))VALUE
We were expecting 10 to be printed, but it’s actually
printing VALUE.
How do we get 10 to be printed
then?
>>> print("VALUE {0}".format(5*2))VALUE 10
By having an open brace
{, closed brace }, and and by putting the index of
the value between them. Here, the value is the first parameter, and it’s index will be
0.
"VALUE
{0}"
is what we need.
Let’s take another example.
Suppose to the format() function, we pass three values:
10, 20 and 30.
Typically when we count positions
or indexes, we start from 0.
To print the first value, you
need to pass in an index of 0. To print the second value, pass
an index of 1.
>>> print("VALUE {0}".format(10,20,30))VALUE 10>>> print("VALUE {1}".format(10,20,30))VALUE 20>>> print("VALUE {2}".format(10,20,30))VALUE 30
Now going back to our problem, we
wanted to display "5 * 6 =
30",
but without hard-coding. Instead of 30, we want the calculated value of
5 *
6.
>>> print("5 * 6 = 30".format(5,6,5*6))5 * 6 = 30
Let replace "5 * 6 =
30"
with "5 * 6 =
{2}".
2 is the index of parameter value
5*6.
>>> print("5 * 6 = {2}".format(5,6,5*6))5 * 6 = 30
Cool! Progress made.
Let’s replace 5 * 6 with the right indices -
{0} *
{1}.
>>> print("{0} * {1} = {2}".format(5,6,5*6))5 * 6 = 30
The great thing about this, is
now we can replace the values we passed to print() in the first place, without
changing the indexes! So, we can display results for 5 * 7 = 35 and 5 * 8 = 40. We are now able to print
5 * 6 =
30,
5 * 7 =
35,
5 * 8 =
40, and
can do similar things for other table entries as well.
>>> print("{0} * {1} = {2}".format(5,7,5*7))5 * 7 = 35>>> print("{0} * {1} = {2}".format(5,8,5*8))5 * 8 = 40>>> print("{0} * {1} = {2}".format(5,8,5*8))5 * 8 = 40
Summary
In this step, we:
Discovered that Python provides a way to do formatted printing of string values
Looked at the format() function, and saw how to
call it within print()
Observed how we could work
only with the indexes of parameters to format(), and change the parameters
we pass without changing the code
In this step, let’s look at a few puzzles related to the format, and the print methods.
Snippet-01: format() And print() Puzzles
Let’s say we pass in additional
values, such as: 5 * 8, 5 * 9 and 5 * 10. However, within the call to
format(), we are only referring to the
values at index 0, index 1 and index 2. The values at indexes
3 and 4 are not used at all. What would
happen when we run the code?
>>> print("{0} * {1} = {2}".format(5,8,5*8,5*9,5*10))5 * 8 = 40
Would this throw an error? No, it does not. You can see that the additional values which are passed in, are conveniently ignored.
Let’s say instead of passing in a
value of 2, we pass 4. What would
happen?
>>> print("{0} * {1} = {4}".format(5,8,5*8,5*9,5*10))5 * 8 = 50
5 * 10 is the value at index
4
Now let’s take a different
scenario. We remove all the parameters passed to format(). However, inside the call to
print(), we continue to say {0} * {1} = {4}. So we are trying to print the
value at index 4, but are only passing two values
to the function format(). What do you think will
happen?
>>> print("{0} * {1} = {4}".format(5,8))Traceback (most recent call last):File "<stdin>", line 1, in <module>IndexError: tuple index out of range
It says IndexError, which means :“you are asking me
to fetch the value at index 4, but only passing in two values.
How can I do what you want?”
Let’s look at a few more things
related to other data types. We try to format the following inside print(): {0} * {1} = {2}, and would pass in 2.5, 2, and 2.5 * 2 . Here, 2 is an integer value, but
2.5 is a floating point value. You
can see that it prints 2.5 * 2 = 5.0. So this approach of formatting
values with print(), works also with floating point
data as well.
>>> print("{0} * {1} = {2}".format(2.5,2,2.5*2))2.5 * 2 = 5.0
Now, are there are other types of
data that format() works with? Yes, strings can
join the party.
Let’s say over here, we do:
print("My name
is {0}".format("Ranga")). What would
happen?
>>> print("My name is {0}".format("Ranga"))My name is Ranga
Index 0 will be replaced with the first
parameter to format().
Summary
In this step, we:
Understood the behavior
when the parameters passed to format():
Exceed the indexes
accessed by print()
Are less than the
indexes accessed by print()
Are of type integer, floating-point or string
We are slowly making progress
toward our main goal, which is to print the 5 multiplication
table.
In the first statement, we are
printing 5 * 1 =
5, and then
changing the literals. To make it print 5 * 2 = 10, we are changing 1 to 2. Next, we are changing
2 to 3. How do we make it a little
simpler, so that our effort is reduced?
>>> print("{0} * {1} = {2}".format(5,1,5*1))5 * 1 = 5>>> print("{0} * {1} = {2}".format(5,2,5*2))5 * 2 = 10>>> print("{0} * {1} = {2}".format(5,3,5*3))5 * 3 = 15
Let’s try a different approach.
What would happen if you replace
1 with index, and 5 * 1 with 5 * index, and try to run
it?
It gives an error! It says: “index is not defined”.
Let’s try and fix this, and
execute index =
2. What
would happen?
>>> index = 2
Aha! This compiles.
>>> print("{0} * {1} = {2}".format(5,index,5*index))5 * 2 = 10
And this statement is printing
5 * 2 =
10.
Let’s try something else. Let’s
make index =
3. What
would happen?
>>> index = 3>>> print("{0} * {1} = {2}".format(5,index,5*index))5 * 3 = 15
The same statement on being run,
prints 5 * 3 =
15.
How can you check the value that
index has? Just type in index.
>>> index3>>> print("{0} * {1} = {2}".format(5,index,5*index))5 * 3 = 15
The index symbol we have used here, is
what is called a variable.
In Python, it’s also called a name.
You can see that the value
index referring to, can change over
the duration of a program.
Initially, index was referring to a value of
1. later, index was referring to a value of
3.
Now, think about how you would
print the entire table. All that you need to do, is start from 1, execute the same statement with
print() and format(), to get output 5 * 1 = 5. Next, Change the value of index
to 2, and then print the same
statement. Next, index = 3, and print the same statement
again.
>>> index = 1>>> print("{0} * {1} = {2}".format(5,index,5*index))5 * 1 = 5>>> index = 2>>> print("{0} * {1} = {2}".format(5,index,5*index))5 * 2 = 10>>> index = 3>>> print("{0} * {1} = {2}".format(5,index,5*index))5 * 3 = 15
With the same statement
print("{0} *
{1} = {2}".format(5,index,5*index)), we are able to print different
values. The value of index varies, but the code remains the
same!
Variables make the program much more easier to read, as well as more generic.
Snippet-02: Classroom Exercise On Variables
Let’s do a simple exercise with variables.
We want to create three variables
a, b and c. Let’s initially give them some
values, say a value of 5 to a, 6 to b and 7 to c.
We want to get output of this
kind: 5 + 6 + 7 =
18, without
using the literal values.
You would want to use the values
stored in the variables in a, b and c.
If you’re hard-coding, the way to
do it is with print("5 + 6 +
7 = 18").
>>> a = 5>>> b = 6>>> c = 7>>> print("5 + 6 + 7 = 18")5 + 6 + 7 = 18>>> print("5 + 6 + 7 = 18".format(a,b,c,a+b+c))5 + 6 + 7 = 18
The way you can do that is with
code like this: print("{0} + {1} + {2} =
{3}".format(a,b,c,a+b+c)).
>>> print("{0} + {1} + {2} = {3}".format(a,b,c,a+b+c))5 + 6 + 7 = 18
How do you confirm we are accessing values stored in the variables?
Let’s change the values of
a, b and c. Let’s make a = 6 , b = 7 , and c = 8 . Execute same
statement.
>>> a = 6>>> b = 7>>> c = 8>>> print("{0} + {1} + {2} = {3}".format(a,b,c,a+b+c))6 + 7 + 8 = 21
You can see the magic of variables at play here! Based on what values these variables are referring to, you can see that the output of the print statement changes.
Summary
In this step, we:
Were introduced to variables, or names, in Python
Observed how we could pass
in values of variables to the format()
function
In the previous step, we were introduced to the concept of variables in Python.
We will start with looking at a few puzzles.
Snippet-01: Puzzles On Variables
What if I try to refer to a variable which is not yet created?
>>> countTraceback (most recent call last):File "<stdin>", line 1, in <module>NameError: name 'count' is not defined>>> print(count)Traceback (most recent call last):File "<stdin>", line 1, in <module>NameError: name 'count' is not defined
Before using a variable, you need
to have it assigned a value. If you have not defined a variable before, then you
cannot use it. Consider print(count), it does not know what count is.
So it would throw an error, saying: “count is not defined, I have no idea
what count is.”
Once you assign a value to a variable, you can use it.
>>> count = 4>>> print(count)4
The statement count = 4 where we are creating a variable
named count for the first time, is called a
variable
definition.
This is the first time you’re referring to a variable, and assigning a value to it.
Python will create a variable in its memory.
Variable names are case
sensitive. count and Count are not the same
thing.
>>> CountTraceback (most recent call last):File "<stdin>", line 1, in <module>NameError: name 'Count' is not defined>>> count4
There are rules to follow while naming variables.
All variable names should either
start with an alphabet , or an underscore _ . count, _count are valid. 1count is invalid.
>>> 1count = 5File "<stdin>", line 11count = 5^SyntaxError: invalid syntax>>> count = 5>>> _count = 5>>> 1countFile "<stdin>", line 11count^SyntaxError: invalid syntax>>> 2countFile "<stdin>", line 12count^SyntaxError: invalid syntax
After the first symbol, you can also use a numeral in variable names.
>>> c12345 = 5
To summarize the rules for naming variables.
This should start with an alphabet (a capital or a small alphabet) or underscore.
Starting the second character, it can be alphabet, or underscore, or a numeric value.
Summary
In this step, we:
Understood that a variable needs to be defined before it is used
Learned that there are certain rules to be followed while giving names to variables
In this step, we will look at an
important concept in Python, called assignment. In previous steps, we created
variables, like i = 5.
Snippet-01: Introducing Assignment
You can create other variables
using whatever value i is referring to. If we say
j =
i, what
would happen?
>>> i = 5>>> j = i>>> j5
j would start referring to the
same value that i is referring to. This statement
is called an assignment.
Let’s try j = 2 * i.
>>> j = 2 * i>>> j10
j refers to a value of
10
= has a different meaning in
programming compared to mathematics.
In mathematics, When we execute
j =
i, it means
j and i are equal.
In prgramming, the value of the expression on right hand side is assigned to the variable on the right hand side. Can you use a constant on the left hand side of an assignment? The answer is “No”!
>>> 5 = jFile "<stdin>", line 1SyntaxError: can't assign to literal
The Python Shell throws an error,
saying “Can’t assign to literal”, as 5 is a
literal.
Let’s create a couple of
variables. num1 =
5 and
num2 =
3. We would
want to add these and create a fresh variable. Let’s say the name of the variable is
sum.
>>> num1 = 5>>> num2 = 3>>> sum = num1 + num2>>> sum8
Create 3 variables a, b and c with different values and
calculate their sum.
>>> a = 5>>> b = 6>>> c = 7>>> sum = a + b + c>>> sum18
We have just seen the mechanics of how assignment works in Python.
Summary
In this step, we:
Learned what happens when you assign a value to a variable, which may or may not exist
Discovered that literal
constants cannot be placed on the left hand side of the assignment(=)
operator
Until now, we have been using the
format() method to format and print
values. Let’s see a better approach to printing values.
This is the approach we used until now.
>>> a = 1>>> b = 2>>> c = 3>>> sum = a + b + c>>> print("{0} + {1} + {2} = {3}".format(a, b, c ,sum))1 + 2 + 3 = 6
Python has the concept of
formatted strings. The syntax to use a formatted string is very simple - f"".
If we want to print the value of
a variable a, we can use {a} in the
text.
>>> print(f"")>>> print(f"value of a is {a}")value of a is 1>>> print(f"value of b is {b}")value of b is 2
The variable within braces is replaced by its value.
You can use expressions in a
formatted string. Example below uses {a+b}.
>>> print(f"sum of a and b is {a + b}")sum of a and b is 3
This feature was introduced in a Python 3 release.
Let’s get back to the original
problem we wanted to solve: printing 5 + 6 + 7 = 18, using formatted
strings.
>>> print(f"{a} + {b} + {c} = {sum}")1 + 2 + 3 = 6
You can see how easy it turns out to be!
We want to print the 5-table from 5 * 1 = 5 onward, until we reach to
5 * 10 =
50. The
best solution we have right now, is shown below:
Snippet-01:
>>> index = 1>>> print("{0} * {1} = {2}".format(5,index,5*index))5 * 1 = 5>>> index = 2>>> print("{0} * {1} = {2}".format(5,index,5*index))5 * 2 = 10>>> index = 3>>> print("{0} * {1} = {2}".format(5,index,5*index))5 * 3 = 15>>> index = 4>>> print("{0} * {1} = {2}".format(5,index,5*index))5 * 4 = 20
Can we do something, to make sure
that the code remains the same all the time, but the index value gets
updated?
>>> index = index + 1>>> print("{0} * {1} = {2}".format(5,index,5*index))5 * 5 = 25>>> index = index + 1>>> print("{0} * {1} = {2}".format(5,index,5*index))5 * 6 = 30>>> index = index + 1>>> print("{0} * {1} = {2}".format(5,index,5*index))5 * 7 = 35
We used index = index + 1 to increment index value.
If we execute these same two statements again and again, we can print the entire table! This is exactly what loops help us do: execute the same statements repeatedly.
The simplest loop available in Python is the for loop.
When we run a for loop, we need to specify the
range of values - 1 to 10 or 1 to 20, and so on. range() function helps us to specify a
range of values.
>>> range(1,10)range(1, 10)
The syntax of the for loop is: for i in range(1, 10):
.... Here,
i is the name of the control variable. In Python, you need to put a
colon, ‘:’, and in the next line give
indentation.
>>> for i in range(1,10):... print(i)...123456789
You would see that it prints from
1 to 9.
When we run a loop in
range(1,
10),
1 is inclusive and 10 is exclusive.The loop runs from 1 to the value before
10, which is 9.
The leading whitespace before
print(i) is called indentation. We’ll talk about indentation
later, when we talk about puzzles related to the for loop.
How can you extend this concept to solving our PMT-Challenge problem?
>>> print(f"{5} * {index} = {5*index}")5 * 7 = 35
What we were doing earlier, was
calling print() with a formatted string. Now we
want to print this statement for different values of i.
How can you do that?
Let’s start with a simple example.
>>> for i in range(1,11):... print(f"{i}")...12345678910
print(f"{i}") prints the value of
i.
Now, how do we get it to print
5 * 1 =
5 to
5 * 10 =
50?
>>> for i in range(1,11):... print(f"5 * {i} = {5 * i}")...5 * 1 = 55 * 2 = 105 * 3 = 155 * 4 = 205 * 5 = 255 * 6 = 305 * 7 = 355 * 8 = 405 * 9 = 455 * 10 = 50>>> 5 * 4 * 501000
print(f"5 * {i} = {5 *
i}")
prints a specific multiple of 5.
In a previous step, we took a major step in programming. We wrote our first for loop with Python. In this step, let’s try a few puzzles to understand the for loop even further.
The syntax of the for loop we looked at earlier was:
for i in range(1, 10):print(i)
Snippet-01:
Let’s say we write a for loop, but don’t give a
: after the range() method, to close the first line.
What would happen?
>>> for i in range(1,10)File "<stdin>", line 1for i in range(1,10)^SyntaxError: invalid syntax
Invalid syntax. A : is mandatory within the
for loop
syntax.
Let’s provide a : and in the next line, use
print(i) without space before it (without
indentation).
>>> for i in range(1,10):... print(i)File "<stdin>", line 2print(i)^IndentationError: expected an indented block
Most other programming languages
use open brace { and closed brace } as delimiters in a for loop. However, Python uses
indentation to identify which code is part of a for loop, and which is not. So if we
are writing the body of a for loop, we must use indentation,
and leave atleast a single <SPACE>.
>>> for i in range(1,10):... print(i)...123456789
How do we execute two lines of
code as part of the for loop?
>>> for i in range(1,10):... print(i)... print(2*i)...12243648510612714816918
We are indenting both statements
with a space - print(i) and print(2*i).
When for loop has only one line
of code, you can specify it right after the :
>>> for i in range(2,5): print(i)...234
However, this is not considered
to be a good programming practice. Even though you may want to execute just one
statement in a for loop, indentation on a new line
is recommended.
Another best practice is to use
four <SPACE>s for indentation, instead of
just two. This would give clear indentation of the code.
>>> for i in range(2,5):... print(i)...234
Anybody who looks at the code
immediately understands that this print() is part of the for loop.
Let’s say you only want to print
the odd numbers till 10, which are 1, 3, 5, 7 and 9. The range() function offers an interesting
option.
>>> for i in range (1,11,2):... print(i)...13579
In for i in range(1, 11,
2), we pass
in a third argument, called a step. After each iteration, the value
of i is increment by step.
Summary
In this step, we:
Looked at a few puzzles
about the for loop, which lay emphasis
on the following aspects of for:
The importance of syntax elements such as the colon
Indentation
Variations of the
range()
function
In the previous step, after
initially exploring the Python for loop, we looked at a number of
puzzles.
In this step, let’s look at a few exercises.
Exercises
Print the even numbers up to 10. We would want to print 2 4 6 8 10, using a for loop.
Print the first 10 numbers in reverse
Print the first 10 even numbers in reverse
Print the squares of the first 10 numbers
Print the squares of the first 10 numbers, in reverse
Print the squares of the even numbers
Solution 1
Instead of starting with
1, we need to start with
2. Each time, i it would be incremented by
2, and 2 4 6 8 and 10 would be
printed.
>>> for i in range (2,11,2):... print(i)...246810
Solution 2
We would want to print the
numbers in reverse. Think about how you would do that using the range() function. We’d want go from
10, 9, 8, and so on up to 1.
>>> for i in range (10,0,-1):... print(i)...10987654321
The value to start with is
10. As we discussed earlier, the
end value is exclusive. So to print from 10 to 1, we want to end one value which
is 0. range(10, 0) seems to be what we
need.
Usually these step value is
positive, but we need to go backwards from 10. Hence, we would give a step
value of -1.
Solution 3
Now, let’s print the first
10 even numbers in
reverse.
>>> for i in range (20,0,-2):... print(i)...2018161412108642
Solution 4
Next, we would want to print the squares of the first 10 numbers.
>>> for i in range (1,11):... print(i * i)...149162536496481100
Solution 5
Let’s print the squares in the reverse order.
>>> for i in range (10,0,-1):... print(i*i)...100816449362516941
Solution 6
Print the squares of the even numbers. How to do that?
>>> for i in range (10,0,-2):... print(i*i)...1006436164
The key part is using a step of
-2
We leave it as an exercise for you, to print squares of odd numbers.
Summary
In this video, we: * Tried out a few exercises involving the for loop, by playing around with printing sequences of numbers.
Used the for loop to simplify the solution to the PMT-Challenge problem.
It must have been a roller-coaster ride to solve the multiplication table challenge so far. If you’re new to programming, there are a wide range of topics and concepts, that you would have learned during this small journey.
Let’s quickly revise the important concepts we have learned during this small journey.
1, 11, 5, … are all called literals
because these are constant values. Their values don’t really change. _Consider
5 _ 4 _ 50`.
This is an expression. `_`is an operator, and`5`, `4`and`50 are
operands.
The name i in i = 1, is called a variable. It
can refer to different values, at different points in time.
range() and print() are in-built Python
functions.
Every complete line of code
is called statement. The specific statement print(), is invoking a method. The
other statement which we looked at earlier, was an assignment statement.
index = index
+ 1
would evaluate index + 1, and have the index variable refer to that
value.
The syntax of the
for loop was very simple.
for var in
range(1, 10) : ..., followed by statements
you would want to execute in a loop, with indentation. For the sake of
indentation we left four <SPACE>s in front of each
statement inside the for loop.
So that, in a nutshell, is what we have learned over the course of our first section.
In the last section, we introduced you to the basics of python. We learned those concepts by applying them to solve the PMT-Challenge problem. The code below is what we ended up with as we solved that chellenge.
Snippet-01: Current Solution To PMT-Challenge
>>> for i in range (1,11):... print(f"8 * {i} = {8 * i}")
If we wanted to change the code
to print the 7 table, we need to change the
value 7 used in the for loop, to
8. It’s simple, but still not as
friendly as you would like.
>>> for i in range (1,11):... print(f"7 * {i} = {7 * i}")
To print a 7 table, it would be awesome if
could say print_multiplication_table, and give a value of 7 beside
it, and it would do the rest:
>>> print_multiplication_table(7)Traceback (most recent call last):File "<stdin>", line 1, in <module>NameError: name 'print_multiplication_table' is not defined>>> print_multiplication_table(8)Traceback (most recent call last):File "<stdin>", line 1, in <module>NameError: name 'print_multiplication_table' is not defined
Similarly, print_multiplication_table(8), could print the multiplication
table for 8!
To be able to do this, we need to create a method, or a function. Creating a method makes the code reusable, and we can invoke that method very easily by passing arguments.
In this section, we take an in-depth look at methods.
Methods are very important
building blocks in Python programming. In this step, we will create a simple method
that prints "Hello
World", twice.
Snippet-01:
When we talk about a method, we
need to give it a name. We are already using an in-built Python method here, which is
print().
>>> print("Hello World")Hello World>>> print("Hello World")Hello World
Similar to that, we need to give
a name to our body of code. Let’s say the name is print_hello_world_twice.
The syntax to create a method in Python is straightforward:
At the start, use the
keyword def followed by a
space.
Followed by name of the
method - print_hello_world_twice.
Add a pair of parenthesis:
().
This is followed by a colon
: (similar to what we used
in a for
loop).
>>> def print_hello_world_twice():... print("Hello World")... print("Hello World")...
All statements in a method should
be indented. The two print("Hello
World") are indented. So, they are part
of the method body.
print_hello_world_twice() defines a method, and it has
certain code inside its body.
How do we call this method? Is it
sufficient to say print_hello_world_twice?
>>> print_hello_world_twice<function print_hello_world_twice at 0x10a71ef28>
Python Shell says, there’s a function defined with that specific name.
How do we execute a method? Very
simple! Add a pair of parentheses to the name, ()!
>>> print_hello_world_twice()Hello WorldHello World>>> print_hello_world_twice()Hello WorldHello World
Now, we are able to run the method.
Summary
In this step, we:
Learned we can define our own methods in the code we write
Understood how to define a method, and all its syntax elements
Saw how we can invoke a method we write
We will now leave you with two exercises, based on what we have learned about methods so far.
Exercises
Write a method called
print_hello_world_thrice(). It should print
"Hello
World" thrice to the output.
Define this method, and also invoke it.
Write and execute a method, that prints four statements:
“I have created my first variable.”
“I’ve created in my first loop.”
“I’ve created my first method.”
“I am excited to learn Python.” You need to print these four statements on four consecutive lines.
Solutions
Solution 1
>>> def print_hello_world_thrice():... print("Hello World")... print("Hello World")... print("Hello World")...>>> print_hello_world_thrice()Hello WorldHello WorldHello World
Solution 2
>>> def print_your_progress():... print("Statement 1")... print("Statement 2")... print("Statement 3")... print("Statement 4")...>>> print_your_progress()Statement 1Statement 2Statement 3Statement 4def print_your_progress():print("Statement 1")print("Statement 2")print("Statement 3")print("Statement 4")
For convenience, we have changed
the exact text we need to print. Call this method with the syntax print_your_progress(), and you’re able to execute its
code.
Now try another exercise. We want
to print "Statement
1",
"Statement
2",
"Statement
3" and
"Statement
4" on
different lines, using just one print statement. How can you do
that?
>>> def print_your_progress():... print("Statement 1\nStatement 2\nStatement 3\nStatement 4")...>>> print_your_progress()Statement 1Statement 2Statement 3Statement 4
We are using the newline
character \n.
Let’s look at the difference between defining and executing a method.
When we are writing a method
definition, we are writing the code as part of its body. It has a specific syntax, and
starts with the def keyword.
A definition by itself cannot cause the code in its body to be executed.
print_your_progress() represents a method call. The
code inside the method is executed.
Summary
In this step, we:
Implemented solutions to a few exercises that test our understanding of Python methods. We touched concepts such as:
Defining a method body
The way to invoke a method, to run its code
The difference between the two
In the previous step,we created
methods. We defined print_hello_world_twice(), and this printed "Hello
World"
twice. In this step, let’s talk about method
arguments, or
parameters.
Snippet-01:
>>> print_hello_world_twice()Hello WorldHello World>>> print_hello_world_thrice()Hello WorldHello WorldHello World
Earlier, we wrote code for
print_hello_world_thrice(), which prints the message three
times.
Let’s say you want to print it five times. You would need to write another method that does what you need. Doesn’t that seem monotonous?
Instead of that, Won’t it be
great if I can call the method by the same name, say print_hello_world(5), and it would print “Hello
World” five times?
The 5 which we are passing here is
called an argument.
How do we define our method to accept this argument?
Let’s call our argument
no_of_times. If you have any experience with
other programming languages, they generally need you to specify the parameter type.
Something like This parameter is an
integer/float/string, or other types. But Python does not require
parameter type.
>>> def print_hello_world(no_of_times):... print("Hello World")... print(no_of_times)...
Although we are not doing exactly
what we set out to, let’s see what would happen. What would happen if we say
print_hello_world() ?
>>> print_hello_world()Traceback (most recent call last):File "<stdin>", line 1, in <module>TypeError: print_hello_world() missing 1 required positional argument: 'no_of_times'
Error! Something like “Hey, you
have created print_hello_world with a parameter, but not
passing anything in here! Go ahead and pass a value”. Let’s pass in a value, such as
5.
>>> print_hello_world(5)Hello World5>>> print_hello_world(10)Hello World10>>> print_hello_world(100)Hello World100
With print_hello_world(5), you can see "Hello
World"
and 5 being printed. We are now able
to define this method to accept a value, and print that value by invoking it. You can
pass in any value, such as10, 100, or
others.
Now think of a different solution
for this method, where you don’t repeat the same piece of code to print "Hello
World". Consider print_hello_world(5), it should still print
"Hello
World"
5 times. How do you do
that?
Think about using something along the lines of a loop.
Snippet-02:
For now, what we are doing is we
are printing "Hello
World"
10 times.
>>> def print_hello_world(no_of_times):... for i in range(1,10):... print("Hello World")...>>> print_hello_world(5)Hello WorldHello WorldHello WorldHello WorldHello WorldHello WorldHello WorldHello WorldHello World
Our method call print_hello_world(5) now prints "Hello
World"
10 times.
However just print the message
5 times. We need to make use of
the parameter no_of_times inside the for loop as
well.
>>> def print_hello_world(no_of_times):... for i in range(1,no_of_times):... print("Hello World")...>>> print_hello_world(5)Hello WorldHello WorldHello WorldHello World
Now let’s execute the method
again. You can see that it’s printing 4 times only.
Why is it not printing
5 times?
That’s because no_of_times as a second parameter to
range() is
exclusive.
>>> def print_hello_world(no_of_times):... for i in range(1,no_of_times+1):... print("Hello World")...>>> print_hello_world(5)Hello WorldHello WorldHello WorldHello WorldHello World
Great, it’s now printing the
message 5 times!
>>> print_hello_world(7)Hello WorldHello WorldHello WorldHello WorldHello WorldHello WorldHello World
If you pass a different argument
like 7, the message is displayed
7 times.
Something you need to always be
cautious about in Python, is the indentation. Over here, the for loop is part of the method body.
So we have extra indentation for it. The print is part of the for loop body. So guess what, even
more indentation for that code.
Summary
In this step, we:
Learned how to pass arguments to a method
Understood that the method definition needs to have parameters coded in
Observed that arguments passed during a method call can be accessed inside a methods body
In this step, Let’s look at a few exercises related to the method parameter.
Exercises
Write a method called
print_numbers(), that would print all
successive integers from 1 to n.
The second one is to write
a method called print_squares_of_numbers(), that prints squares of
all successive integers from 1 to n.
Solutions
Solution 1
>>> def print_numbers(n):... for i in range(1, n+1):... print(i)...>>> print_numbers(5)12345>>>
If you are programming in other
languages such as Java, you are used to naming methods in this way: printNumbers(). This convention is popularly
known as “Camel Case”.
That’s NOT how Python programmers
name their methods. Pythonic way is to use underscore _ to separate words in the method
name, as in print_numbers().
Solution 2
Let’s define print_squares_of_numbers(). This would be very similar to
print_numbers(), working with the same range.
Only, we need to say print(i*i) .
>>> def print_squares_of_numbers(n):... for i in range(1, n+1):... print(i*i)...>>> print_squares_of_numbers(5)1491625
How is a parameter different from an argument?
Inside the definition of
the method, the name within parentheses is referred to as a parameter. In our recent exercise,
n is a parameter, because
it’s used in the definition of print_squares_of_numbers.
When you are passing a
value to a method during a method call, say 5, that value is called an
argument.
Don’t worry too much about it. Just follow this convention for now:
In the method call, call it an argument.
In a method definition, call it a parameter.
Summary
In this step, we looked at a few simple exercises related to passing method arguments
In this step, let’s look at creating a method with multiple parameters.
Snippet-01:
print_hello_world accepts one parameter and prints
“Hello World” the specified number of times.
>>> def print_hello_world(no_of_times):... for i in range(1,no_of_times+1):... print("Hello World")...
Let’s say we want to print
another piece of text Welcome To Python, a specified number of times.
How do you do that?
You can always create another
method similar to the first one, such as print_welcome_to_python(no_of_times) and print the necessary text
inside.
However, is that what a good programmer does?
A good programmer tries to create a more generic solution.
>>> def print_string(str, no_of_times):... for i in range(1,no_of_times+1):... print(str)...>>> print_string("Hello World", 3)Hello WorldHello WorldHello World
The good programmer that you are,
you created a new method called print_string(str,
no_of_times) accepting a text parameter, in
addition to no_of_times.
Syntax rules for method
parameters are quite strict. If we say print_string("Welcome to
Python") and run it, we get an error!
Python Shell says: “I need no_of_times to be present in
here”.
>>> print_string("Welcome to Python")Traceback (most recent call last):File "<stdin>", line 1, in <module>TypeError: print_string() missing 1 required positional argument: 'no_of_times'
Let’s say you want to assign
default values for str and no_of_times in print_string(). By default, we want to always
print "Hello
World", and that too 5 times.
The Python language makes this
very easy. def print_string(str
= "Hello World", no_of_times=5). The rest of the method remains
the same.
>>> def print_string(str="Hello World", no_of_times=5):... for i in range(1,no_of_times+1):... print(str)...
Now you can call print_string(), and "Hello
World"
is displayed 5 times.
>>> print_string()Hello WorldHello WorldHello WorldHello WorldHello World
If it’s print_string("Welcome To
Python"), what does it do? It prints
"Welcome To
Python", 5 times.
>>> print_string("Welcome to Python")Welcome to PythonWelcome to PythonWelcome to PythonWelcome to PythonWelcome to Python
Consider print_string("Welcome to
Python", 8), it would print that string
8 times.
>>> print_string("Welcome to Python", 8)Welcome to PythonWelcome to PythonWelcome to PythonWelcome to PythonWelcome to PythonWelcome to PythonWelcome to PythonWelcome to Python
Isn’t that cool!
Summary
In this step, we:
Looked at how to pass multiple parameters to a method, starting with two arguments
Learned how you can define default values for those parameters
Observed we could pass default arguments for none, some or all of those parameters
Let’s get back to our original goal, of why we needed methods. We wanted to create a multiplication table for a number, and observed that each time we needed to we needed change that number, we were forced to make a change in the code. This is not something we liked, and that’s why we started investigating how methods can be used.
In this step, Let’s try our hand at creating a multiplication table method.
Snippet-01:
>>> for i in range (1,11):... print(f"7 * {i} = {7 * i}")
Let’s define a method called
print_multiplication_table(), and pass in a parameter to
it.
>>> def print_multiplication_table(table):... for i in range(1,11):... print(f"{table} * {i} = {table * i}")...>>> print_multiplication_table(7)7 * 1 = 77 * 2 = 147 * 3 = 217 * 4 = 287 * 5 = 357 * 6 = 427 * 7 = 497 * 8 = 567 * 9 = 637 * 10 = 70
Now you have the entire
multiplication table for 7.
You can then call print_multiplication_table() with arguments 8, 9,and so on, by simply changing
the table arguemnt
value.
We now want to create even better
print_multiplication_table() method.
We want to control the start
point, as well as the end point, in the call to range(). We want to say print_multiplication_table(7, 1,
6), to
print the 7 table with entries from
1 to 6. How can you do
that?
>>> def print_multiplication_table(table, start, end):... for i in range(start, end+1):... print(f"{table} * {i} = {table * i}")...>>> print_multiplication_table(7, 1 , 6)7 * 1 = 77 * 2 = 147 * 3 = 217 * 4 = 287 * 5 = 357 * 6 = 42
Simple! Define those range limits as additional parameters!
The other thing we can obviously
do, is have default values for the start, and the end.
>>> def print_multiplication_table(table, start=1, end=10):... for i in range(start, end+1):... print(f"{table} * {i} = {table * i}")...>>> print_multiplication_table(7)7 * 1 = 77 * 2 = 147 * 3 = 217 * 4 = 287 * 5 = 357 * 6 = 427 * 7 = 497 * 8 = 567 * 9 = 637 * 10 = 70
Calling print_multiplication_table(7) would give us entries from
7 * 1 =
7 to
7 * 10 =
70.
Now you can actually send out this method, to your friends, who would find it easy to use, and cool!
Summary
In this step, we:
Learned how to define a method to print the multiplication table for a number
Looked at how to enhance this method to make table printing more flexible
Further enhanced that method to accept default arguments while printing a table
In Python, indentation denote
blocks of code. So if you want to put something in a for loop, or outside it, proper
indentation would be sufficient. In this step, let’s explore indentation in depth.
Let’s start by creating a simple method.
Snippet-01:
>>> def method_to_understand_indentation():... for i in range(1,11) :... print(i)...>>> method_to_understand_indentation()12345678910
Consider the code below:
print(5) is indented at the same level as
for
loop.
>>> def method_to_understand_indentation():... for i in range(1,11) :... print(i)... print(5)...
You can see that print(5) is called only once. It is not
part of the for
loop.
>>> method_to_understand_indentation()123456789105
Let’s change the code in this
method a bit. print(5) is indented the same way as
print(i)
>>> def method_to_understand_indentation():... for i in range(1,11) :... print(i)... print(5)...
print(5) is part of the for loop. It is
executed 10 times.
>>> method_to_understand_indentation()152535455565758595105
Whether we’re talking about loops, methods or conditionals, proper indentation is very important in Python.
We indicate a block of code, by
having all lines of that block at the same indentation level. There are no specific
delimiters like for instance a pair of braces {...}, as in other programming
languages.
Summary
In this step, we:
Ran through a few examples to see how indentation works in Python
In this step, let’s look at a variety of puzzles related to methods.
Snippet-01:
Consider the following method: I would want to print the default string 6 times. How do we do it?
>>> def print_string(str="Hello World", no_of_times=5):... for i in range(1,no_of_times+1):... print(str)...>>> print_string()Hello WorldHello WorldHello WorldHello WorldHello World
Will it work if we call the
method as in: print_string(6)?
>>> print_string(6)66666
6 is passed as the first
parameter. 6 is matched to str, and the method prints
6 the default number of times,
which is 5.
to default to "Hello
World", and print it 6 times.
You can do this in Python by
using named parameters. During the method call, you can
specify no_of_times =
6.
no_of_times is a named
parameter.
There is no provision of doing something like this, in other languages like Java.
Call it as print_string(no_of_times=6):
>>> print_string(no_of_times=6)Hello WorldHello WorldHello WorldHello WorldHello WorldHello World
str gets a default value, and
"Hello
World"
is printed 6 times.
Named parameters are very useful, when a method has a number of parameters, and you would want to make it very clear which parameter you’re passing a value for.
Let’s call print_string(7, 8). what
happens?
>>> print_string(7, 8)77777777
You would see that 7 is printed 8 times.
Since print() method is quite flexible, you
can pass a number as the first argument. You can even pass a float.
>>> print_string(7.5, 8)7.57.57.57.57.57.57.57.5
What would be the result of this
- print_string(7.5,
"eight")?
>>> print_string(7.5, "eight")Traceback (most recent call last):File "<stdin>", line 1, in <module>File "<stdin>", line 2, in print_stringTypeError: must be str, not int
Note how no_of_times is used inside the method… as an
argument to range(). range() only accepts integers, nothing
else. When you run the code with print_string(7.5,
"eight"), we get an
error.
It says: TypeError: ```no_of_times``` must be
```int```, not string.
A simple rule of thumb is, if you
have a parameter, you can pass any type of data to it. That could be an integer, a
floating point value a string, or a boolean value. The Python language does not check
for the type of a parameter. However, Python will throw an error if the function which
is using that parameter, expects it to be of a specific type. The range() function expects that the
no_of_times is an integer
value.
Snippet-02:
The last thing which we would be
looking at, is method naming conventions. We named our methods in a consistent way:
print_string, print_multiplication_table, and the
like.
This is exactly the format which most Python developers use, to name their methods.
Convention is to use underscore to separate words in a name.
However, there are a few rules for naming a method: One of the important rules is also related to variable names. We observed that a variable name cannot start with a number.
>>> def 1_print():File "<stdin>", line 1def 1_print():^SyntaxError: invalid token
Similarly, 1_print will not be accepted as a method
name.
You can start a name with an alphabet, or with an underscore.
From the second character onward, you are allowed to use numeric symbols.
Methods and variables cannot be named using Python keywords.
Now, what is a keyword? For
example, when we talked about for loop, as
in:
```for i in range(1, 11): print(i)```...
for is a
keyword
in is a
keyword
def is a
keyword.
Later we will look at a few other
keywords, such as while, return, if, else, elif, and many
more.
>>> def def():File "<stdin>", line 1def def():^SyntaxError: invalid syntax>>> def in():File "<stdin>", line 1def in():^SyntaxError: invalid syntax>>> def for():File "<stdin>", line 1def for():^SyntaxError: invalid syntax
Summary
In this step, we:
Were introduced to the concept of named parameters
Explored the typical naming rules and conventions for methods in Python
Observed that reserved keywords cannot be used to name variables or methods
Let’s try and understand the importance of return values from a method. We will learn how to return a value from a method.
Snippet-01:
Let’s name our method as
product_of_two_numbers(), and let’s have parameters
a and b that it
accepts:
>>> def product_of_two_numbers(a,b):... print(a * b)...>>> product_of_two_numbers(1,2)2
Can we take the product of these two numbers into a variable, and use it in other code, in the same program?
Suppose we say a product =
product_of_two_numbers(1,2), is this
allowed?
Let’s run this code, and see
what’s stored in product.
>>> product = product_of_two_numbers(1,2)2>>> product
It’s empty.
The product_of_two_numbers() method is not really returning
anything back, to be used elsewhere.
Have a look at some of the
built-in Python functions, such as max() for
example.
>>> max(1,2,3)3>>> max(1,2,3,4)4>>> maximum = max(1,2,3,4)>>> maximum4>>> maximum * 520
If I call max() with four parameters, as in
maximum =
max(1,2,3,4), the value 4 gets stored in
maximum.
Later on in the code that
follows, we can say maximum * 5, or we can print the value of
maximum, or a similar calculation. This
gives our programs a lot more flexibility.
So instead of just printing
a*b, if this function could return a
value, that would be quite useful.
>>> def product_of_two_numbers(a,b):... product = a * b;... return product...>>> product_of_two_numbers(2,3)6
We are creating a variable
product and doing a return product.
Lets run product_result =
product_of_two_numbers(2, 3)
>>> product_result = product_of_two_numbers(2,3)>>> product_result6>>> product_result * 1060
You can see how simple it is to return values from a method!
Summary
In this step, we:
Learned how to return values from inside a method
Observed how we can store the values returned by a method call
In this step let’s look at a couple of exercises about returning values from methods.
Exercises
Write a method to return the sum of three integers.
Write a method which takes as input two integers, representing two angles of a triangle, and computes the third angle.
Hint: The sum of the angles in a
triangle is 180 degrees. So if I am passing
50 and 50, 50 plus 50 is 100. So some of three angles should
be 180, so the third angle will be
180 -
100, which
is 80.
Solution 1
>>>def sum_of_three_numbers(a, b, c):... sum = a + b + c... return sum...>>> sum_of_three_numbers(1,2,3)6>>> something = sum_of_three_numbers(1,2,3)>>> something * 530
The shorter way of doing that
would have been to have a temporary variable called instead of sum. We could directly return a + b + c.
>>> def sum_of_three_numbers(a, b, c):... return a + b + c...>>> something = sum_of_three_numbers(1,2,3)>>> something * 530
In methods, you can use
return
expression
as well. That expression gets evaluated, and the value
gets returned back. You’d see that the result remains the same.
Solution 2
The second is to write a method to take two integers, representing two angles of a triangle, and compute the third one.
>>> def calculate_third_angle(first, second) :... return 180 - ( first + second )...>>> calculate_third_angle(50, 20)110
In your programming career, you would be writing a number of methods. It’s very important that you are comfortable doing so. Most of the methods that you write would return values back.
That’s the reason why we’re creating a lot of examples involving method calls.
Summary
In this step, we:
Looked at a couple of exercises related to returning values from methods
Observed that returning expressions avoids creating unnecessary variables, and shortens method definitions
Until now we had been using Python Shell to execute all our code.
In the real world, we’ll be write Python code in a variety of scripts. Before we would go into an IDE and use the IDE to write the script, we thought it would be useful for us to understand how you can write Python code without the benefit of an IDE.
This would also help us understand the Python environment, in-depth.
In the next few steps, we’ll be looking at how to create simple Python scripts, using any text editor of your choice. Use Notepad, Notepad++. Editpad, or whichever text editing software you are comfortable with. We’ll see what involved in executing the program, and what’s happening in the background.
Here are a few videos you might want to look at.
Here’s a recommended video to watch - Writing and Executing your First Python Script
Let’s get started with creating a simple script file.
We want to type in a simple
Python script, or a piece of Python code, such as print("Hello
world"). Does it get any simpler than
this?
We’ll save this into any folder on our hard disk, with a name ‘first.py’ .
first.py
print("Hello world")
The ‘.py’ is not really mandatory, but typically all python files end with a ‘.py’ extension.
Here’s how you can run it:
Launch your terminal, or command prompt
‘cd’ to the folder where this python script file is saved
execute the command
python
first.py
You will see that Hello World will be
printed.
If you are familiar with other programming languages, you would need a class, need to put the code in that class, and similar stuff.
While Python supports Object Oriented Programming, is not mandatory to create a class.
It’s almost as if you’re typing commands, starting from the line one! That’s why we call it a python script.
Summary
In this small step, we tried to create a simple python script, and we ran it from the command line. All we needed to do, was use the same command we use to launch up the python shell, and followed it up with a name of the file. We created a file called first.py, executed that, and were able to see the output on the console.
As an exercise, try and add a few more methods and try to run those methods as well, as part of this script.
In this step, let’s try and understand what’s happening in the background.
We wrote a simple piece of code
using a text editor. We created a file named first.py, and all we did was:
python3
first.py.
If you look at other languages like Java for example, there is a separate compilation
phase and then an execution phase. But with Python, just this command does both
compilation and execution.
We saw that, as soon as we make a
change and we run python3 first.py , the change is compiled and
executed as well!
In Python, there is an intermediate format called Python byte code. Code is first compiled to bytecode, and then executed on the Python virtual machine.
When we installed Python, we installed both the python compiler and interpreter, as well as the virtual machine.
In Python, bytecode is not standardized. Different
implementations of Python have different byte code. There are about 80 Python
implementations, like CPython and Jython.
CPython is a Python implementation in C language.
Jython is a Python implementation in Java language. The bytecode which Jython uses is actually Java bytecode, and you can run it on the Java virtual machine.
Python leaves a lot of flexibility to the implementations of Python. They have the flexibility to choose the bytecode, and to choose the virtual machine that is compatible. The bytecode is tied to the specific virtual machine you are using. Therefore, if you’re using CPython to compile the bytecode, you’ll not be able to use Jython to run it.
You should make sure, that whatever implementation you are using to compile, is the same one you’re using to run the code as well.
Summary
A lot of this sounds like boring theory. Don’t worry about it. As a beginner, this might not be very important for you right now.
It’s very important for you to
understand the process. What’s happening is you were writing Python code, and when you
ran the command python3 first.py, it is both compiled and
executed. An intermediate format called bytecode is created, which is not really
standardized in Python. The bytecode is executed in a Python virtual
machine.
The idea behind this quick section, is to give you a little bit of background on what’s happening behind the scenes. I’ll see you in the next section. Until then, bye-bye!
Let’s start using the IDE VSCode to write our Python Code
Here are recommended videos to watch
In this quick step, we’ll help you install VSCode.
Here’s the video guide for this step
Go to Google and type in “VSCode Community Edition Download”. Click the link which comes up first: https://www.jetbrains.com/VSCode/download.
You’ll go to a page where you can choose the operating system: whether you are on Windows, Mac, or Linux.
Once you choose that, you can download the appropriate community version.
On the right hand side, you’ll see a community version, and you can click the download link, to start the download.
If you are having a problem, you can also use the direct link to download.
Once you download VSCode, all you need to do is double-click the package which is downloaded. Follow the instructions, and you can continue with the defaults, until you completely install VSCode.
When you launch VSCode for the first time, it should ask you for a theme, where you can choose the default.
You’re all set to go ahead with the next step in the course.
VSCode is an awesome IDE, and I’m sure you learn a lot about it.
In this step, let’s launch up the VSCode IDE, and create our first Python project with a Python script. We want to be able to launch a Python script by the end of this step.
Here’s the video guide for this step
Launch the VSCode IDE. You’ll see that it takes a little while to launch the first time, and then brings up a welcome screen.
We would want to create a number of Python files. All these files will be in a project. You can think of our project as a collection of Python scripts, or modules.
To get started, let’s create a new project by clicking ‘create new project’. Let’s name it - ‘01-first-python-project’.
Right now there are no files in the project.
Let’s create our first Python file, using the IDE.
The way you can do that is by saying ‘right-click’ -> ‘new’ -> ‘Python file’, and then we’ll give this a name of ‘hello_world’, and click OK.
Now you can go ahead and write
your first Python program. Let’s write some simple code, like print("Hello
World"), and save
it.
You can do a right-click here, and say ‘Run hello_world’.
A small window comes up below,
which shows the output. It says 'Hello
World'.
Let’s start with a simple exercise. We created the multiplication table method in the Python Shell. What we do now, is we’ll create the same thing but in a Python file of its own.
Here’s the video guide for this step:
Welcome to this section, where we will talk about numeric data types, and conditional program execution. After looking at the numeric and boolean data types, we will turn our attention to executing code, based on logical conditions.
In previous sections, we created
variables of this kind: number = 5 , value = 2.5, etc. The 5 here is an integer, and integers
represent numbers, such as 1, 2, 6, -1 and -2. In Python, the class for this particular data type
is int.
If you write code like
type(5), you’d get 'int' as the
output.
In Python, there are no primitive
types. What does that mean? Every value that you see in a Python program, is an
object, an instance of some class.
In later sections, We’ll
understand what is a class, and what is an object or an
instance. For now, the most important thing for you to remember, is that behind every
value, there is a class.
Snippet-01:
Let’s look at 2.5, which is a floating point
value.
If you go ahead and do
type(2.5), what would you see? You would
see it’s of type `float.
>>> type(2.5)<class 'float'>>>> type(2.55)<class 'float'>
When you perform a division
operation between two integers, there is a chance that the result of the operation is
a float. If you do 5/2, the result is 2.5. If we were to do 4/2, even then it’s of type
float.
>>> type(5/2)<class 'float'>>>> type(4/2)<class 'float'>>>> 4/22.0>>> 1 + 23
All the operations we looked at until now, can also be performed on floating point values.
>>> value1 = 4.5>>> value2 = 3.2>>> value1 + value27.7>>> value1 - value21.2999999999999998>>> value1 / value21.40625>>> value1 % value21.2999999999999998
value1 - value2 returns 1.299999999999998. Why?
Floating point numbers don’t really represent accurate values. That’s one of the things you need to always keep in mind.
Typically, if you’re doing any
highly sensitive financial calculations, don’t use floats to represent your values.
Instead, use Decimal. More about it
later.
Operations can also be performed
between int and float.
>>> i + value114.5>>> i - value15.5>>> i / value12.2222222222222223>>>
Result of an operation between a
int and a float, is always a float.
Summary
In this step, we:
Looked at the two basic
numeric types: int and float.
Saw the basic operations
you can do among ints, among floats, and also between
ints and floats.
In this step, let’s do a simple exercise with numeric values.
Exercises
You need to create a method
called simple_interest, and pass three
parameters: principal, interest and duration (in years). You also want
to calculate the amount after the specific duration, and return it back. Call
this method with a few example values.
For example, if you want to call
simple_interest with 10000, with an interest of
5 percent, for a duration of
5 years, the correct answer would
be as follows: 10000 is the principal. In addition
to 10000, you get the interest. The
interest for one year is 10000 * 0.05, as the interest figure is in
percentage.So that’s 500 a year, into 5 which is 2500. The result would be
12500, and this value should be
printed.
Solution 1
def calculate_simple_interest(principal, interest, duration) :return principal * (1 + interest * 0.01 * duration)print(calculate_simple_interest(10000,5,5))
Summary
In this step, we:
Wrote a very simple method to do a simple interest calculation
In this section, we are looking at numeric types. In this specific step, we would be looking at a few puzzles related to values of these types.
Snippet-01:
Let’s create a simple variable
i =
1.
i = i +
1. What
would be the value of i after that?
>>> i = 1>>> i = i + 1>>> i2
It would be 2. There is a shortcut way of
doing the same thing, by using the += operator.
>>> i += 1>>> i3
Typically in other programming
languages, you can do something of this kind: i++. There is no provision in Python
to use increment operators like ++, in either prefix or suffix
mode, like ++i, or i++.
>>> i++File "<stdin>", line 1i++^SyntaxError: invalid syntax>>> ++i3
Let’s look at compound assignments.
>>> i += 1>>> i4>>> i -= 1>>> i3>>> i /= 1>>> i *= 2>>> i6.0
What you see here, is Dynamic Typing in Python. The type of a variable can change during the lifetime of the program.
>>> i = 2>>> type(i)<type 'int'>>>> i = i/2.0>>> type(i)<type 'float'>
Let’s create a couple more
numbers. number1 =
5 and
number2 =
2. What
could be the result of number1 / number2? You know it, it’s 2.5 .
number1 // nummber2 truncates the value of
2.5, to 2.
>>> number1//number22
If you can do number1 // number2, can you also do this:
number1 //=
number2?
>>> number1 //= 2>>> number12
5 ** 3 is 5 ‘to the power of’ 3, which is 5 * 5 * 5, or 125.
>>> 5 ** 3125>>> pow(5,3)125
This can also be achieved by
invoking pow(5,
3). We have
an operator, as well as a method at our disposal.
The last thing we will look at, are type conversion functions.
If you need to convert an
int value to a float, or a float to an int.
>>> int(5.6)5
What if you want to round a
value? 5.6 is nearer to 6 than 5. You can use a function called
round(), and here,round(5.6) gives the correct result
6.
>>> round(5.6)6>>> round(5.4)5>>> round(5.5)6
round() can also allows you to specify
number of decimals in the result.
>>> round(5.67, 1)5.7>>> round(5.678, 2)5.68
You can also convert int to float, by using the function
float().
>>> float(5)5.0
Summary
In this step, we:
Looked at a few corner cases related to your numeric types.
Examined the different operators available for use with values of numeric types
Learned about the usage of type conversion functions
We will now shift our attention
to the bool data type.
A boolean value is something which can be either “true” or “false”.
Snippet-01:
In Python, “true” is represented
by True, and “false” by False. It’s important to remember that
it’s True with a capital 'T', and False with a capital 'F'.
>>> TrueTrue>>> FalseFalse>>> trueTraceback (most recent call last):File "<stdin>", line 1, in <module>NameError: name 'true' is not defined>>> falseTraceback (most recent call last):File "<stdin>", line 1, in <module>NameError: name 'false' is not defined
The boolean variable is_even indicates whether a number is
even or not.
>>> is_even = True>>> is_odd = False
Let’s create a variable
i =
10. We want
to find out if i >
15. What do
you think is the result? False.
>>> i = 10>>> i > 15False>>> i < 15True
In general, boolean values can represent the result of logical conditions.
Let’s look at other operations
that can result in bool values. We looked at
> and <. Another operation which you can
perform, is >=.
>>> i >= 15False>>> i >= 10True>>> i > 10False>>> i <= 10True>>> i < 10False
== is the comparison operator. We
are only comparing the value of i against 10, not changing its
value.
>>> i == 10True>>> i == 11False
Summary
In this step, we:
Were introduced to the
bool data
type
Learned that bool variables are useful handy
while testing logical conditions
In this step, let’s look at
if statement.
Sometimes you need to execute
code only when certain conditions are true. You can use a if condition, which is the simplest
conditional in Python. Let’s look at an example.
Snippet-01:
Let’s say i has a value of 5. You want to print something,
only if i has a value greater than
3. How do you do
that?
>>> i = 5>>> if i>3:... print(f"{i} is greater than 3")...5 is greater than 3
The syntax of the if is very simple: if followed by a condition; with
the condition you want to check. It looks like: if i>3: ... You need to indent the body of
the if with <SPACE>s as usual.
Let’s say i has a value of 2. What would happen if we execute
the same code again?
>>> i = 2>>> if i>3:... print(f"{i} is greater than 3")...
You would see that nothing is
printed to the console. Based on the value of i , either the statement is
executed, or it’s not. That’s what an if helps us to
do.
The way you can think about an
if, is the body of code under the
if is executed only when this
condition is True. If this condition is not
True, that code is not executed at
all.
>>> if(False):... print("False")...>>> if(True):... print("True")...True
Let’s take two different numbers,
say a =
5, and
b =
7. We want
to compare them, and predict if a is greater that b .
>>> a = 5>>> b = 7>>> if(a>b):... print("a is greater than b")...>>> a = 9>>> if(a>b):... print("a is greater than b")...a is greater than b
Summary
In this step, we:
Were introduced to the
if statement, the simplest
Python conditional
Understood how an
if helps in implementing
conditional program logic
In this step, let’s look at a couple of exercises with the if statement.
Snippet-01:
Let’s say we define four
variables: a =
1,
b =
2 ,
c =
3 and
d =
5. we want
to find out, if a + b is greater than c + d.
>>> a = 1>>> b = 2>>> c = 3>>> d = 5>>> if a+b > c+d :... print("a+b > c +d")...>>> a = 9>>> if a+b > c+d :... print("a+b > c +d")...a+b > c +d
Let’s say we are given three
values meant to be the angles of a triangle. Their values are angle1 = 30, angle2 = 20 and angle3 = 60. You want to find out if these
three angles actually form a valid triangle. You know that the sum of the angles of a
triangle is always 180 degrees.
>>> angle1 = 30>>> angle2 = 20>>> angle3 = 60>>> if(angle1 + angle2 + angle3 == 180):... print("Valid Triangle")...>>> angle2 = 90>>> if(angle1 + angle2 + angle3 == 180):... print("Valid Triangle")...Valid Triangle
The last exercise is to check if a number is even or not.
Hint L you need to use one of the
operators we talked about earlier. That’s right, use the modulo operator %.
>>> i = 2>>> if(i%2==0):... print("i is even")...i is even>>> i = 3>>> if(i%2==0):... print("i is even")...
Summary
In this step, we:
Looked at a few exercises related to the if statement, for writing and testing conditions.
In this step, let’s look at the
different operators that can be used on bool values. These operators are
called logical operators - and, or , not and ^ (xor).
Let’s say we have a value
True, and the other False, and we want to play around with
them.
Logical operator and returns true only when both
operands are True.
>>> True and FalseFalse>>> True and TrueTrue>>> True and FalseFalse>>> False and TrueFalse>>> False and False
Logical operator or returns true when atleast one of
the operands is True.
False>>> True or FalseTrue>>> False or TrueTrue>>> True or TrueTrue>>> False or False
Logical operator not returns
negation.
False>>> not TrueFalse>>> not(True)False>>> not FalseTrue>>> not(False)True
The XOR operation, denoted by the
^ operator, is True when operands have different
boolean values.
>>> True ^ TrueFalse>>> True ^ FalseTrue>>> False ^ TrueTrue>>> False ^ FalseFalse
Summary
In this step, we:
Looked at the logical
operators that act on boolean values, such as and, or, not and ^
Explored each of these
operators, finding out when they return True, and when False.
In this step, Let’s look at a few simple puzzles to look at the logical operators.
Snippet-01:
Let’s say i has a value of 10, and j has a value of 15. You want to find out if both
i and j are even. How do you do
that?
>>> i = 10>>> j = 15>>> if i%2==0 and j%2==0:... print("i and j are even")...>>> j = 14>>> if i%2==0 and j%2==0:... print("i and j are even")...i and j are even>>> if i%2==0 or j%2==0:...File "<stdin>", line 2^IndentationError: expected an indented block>>> if i%2==0 or j%2==0:... print("atleast one of i and j are even")...atleast one of i and j are even
If we want to find out if at
least one of i and j is even, we can use the
or operator.
>>> i = 15>>> j14>>> if i%2==0 or j%2==0:... print("atleast one of i and j are even")...atleast one of i and j are even>>> j = 23>>> if i%2==0 or j%2==0:... print("atleast one of i and j are even")...>>> i15
Now try and guess the value of
this. if(True ^ False):
print("Message")
>>> if(True ^ False):... print("This will Print")...This will Print>>> if(False ^ True):... print("This will Print")...This will Print>>> if(True ^ True):... print("This will Print")...
Xor operation using ^ - message will get printed if
the operands are different.
What would happen if both of them
are True? No message is
printed.
So you would use ^ in situations, where you’d want
one of the operands to be True, and the other to be
False.
Let’s say, x = 5, and you want to check
if not x == 6:
print("This"). What will be the result of
running this code?
>>> x = 5>>> if not x == 6:... print("This")...This>>> x = 6>>> if not x == 6:... print("This")...
Actually, there is a shortcut for
such a condition: if x != 6 :
print("This").
>>> if x!=6:... print("This")...>>> x=5>>> if x!=6:... print("This")...This
int() is a conversion function, which
when given say a float value, returns an int value. Consider int(True), what would
happen?
>>> int(True)1>>> int(False)0
int(True) returns 1. int(False) returns 0.
>>> x = -6>>> if x:... print("something")...something
One of the most interesting facts
about boolean stuff, is anything which is non-zero, is considered to be True.
0 is the only integer value which
is considered to be False.
>>> bool(6)True>>> bool(-6)True>>> bool(0)False>>>
So, if I have a value of
x =
-6, and
execute if x:
print("something") what do you think will
happen?
"something" will be
printed.
You can use the function
bool(), to convert int to a bool value.
bool(6) returns True
bool(-6) returns True
bool(0) returns False.
Except for bool(0), all the other results would be
True.
Summary
In this step, we:
Looked at a few puzzles related to the logical operators
Looked at conversion
functions such as bool() and int() to convert between boolean
and integer data
In this step, let’s look at two
other important components of an if statement: else and elif. Let’s start with else.
Snippet-01:
Consider a scenario where
i has a value of 2. Let’s try to print a message
"i is
even"
if i is an even number. Otherwise,
print "i is
odd".
Earlier we wrote code along these
lines: if i % 2 == 0 :
print("i is even"). However if this condition is
not True, we would want to print("i is
odd").
How do we accomplish that?
>>> i = 2>>> if i%2 == 0:... print("i is even");... else:... print("i is odd");...i is even
An else clause provides an alternative
code body to execute, if the if condition is False.
>>> i = 3>>> if i%2 == 0:... print("i is even");... else:... print("i is odd");...i is odd
Let’s look at elif.
We want to do something if
i has value of 3, and something totally different
if i has a value of 4.
In short, we want to specify 2
alternatives to the if condition. How can that be
done?
>>> if i==1:... print("i is 1")... elif i==2:... print("i is 2")... else:... print("i is not 1 or 2")...i is not 1 or 2>>>
That’s where the elif clause comes into the picture.
The code in elif is executed if the previous
conditions are false and the current elif condition is
true.
Summary
In this step, we:
Looked at two important
components of the if statement: else and elif.
Understood that the
elif clauses and the final
else clause provide alternative
conditions to check, when earlier if conditions are true.
In this step, let’s do a simple
exercise with if, else and elif.
Before getting to the exercise, let’s try and learn how to get console input from the user.
Until now, we had been hard-coding all the data we were to use. Let’s make that part more dynamic now.
Snippet-01:
How do we get input from the
user? We want to get input from the console, and assign it to a variable. The way we
can do that, is by statement value =
input()
value = input("Enter a Value: ")print("you entered ", value)
We can call the input() method with a text ‘prompt’,
such as "Enter A Value:
".
What we can initially do here, is print the value which was entered, back to the
console, by print("you
entered ", integer_value).
An interesting point to explore here, is the type of data input at the console.
Let’s do a print(type(value)).
value = input("Enter a Value: ")print("you entered ", value)print(type(value))
Input a value of Test. It has a class of str.
Let’s run it again to see other
possibilities. This time, let’s enter a numeric value, say 12. what would
happen?
We again get str.
We want to get an integer value from the input. How can we do it?
int() function converts string to int.
Let’s use it.
value = input("Enter a Value: ")integer_value = int(value)print("you entered ", integer_value)print(type(integer_value))
Let’s run our code once again.
"Enter A Value:
" is
prompted, and we enter 15. And now, of it says
"You entered
15",
and the type it indicates to us, is int.
Design a menu
Ask the User for input:
Enter two numbers
Choose the Option:
1 - Add
2 - Subtract
3 - Multiply
4 - Divide
Perform the Operation
Publish the Result
Let’s design a menu, and then ask the user for input.
We have codes for each of the
operations : add is 1, subtract is 2, divide is 3, and multiply is 4.
In the first version of the program let’s get all the inputs and print them out.
Solution
The first version of the program is simple to write
number1 = int(input("Enter Number1: "))number2 = int(input("Enter Number2: "))print(f"You entered {number1}")print(f"You entered {number2}")print(number1 + number2)print("\n\n1 - Add")print("2 - Subtract")print("3 - Divide")print("4 - Multiply")print("5 - Exit")choice = int(input("Choose Operation: "))print(choice)
We will continue this exercise to complete it, in the next step.
Summary
In this step, we:
Looked at the in-built
input() function that can read
console input
Learned that input() always returns what the
user enters, as a string
We can convert the string
from input(), to the data type we
expect by invoking conversion functions
Exercises
In the previous step, we got the input from the user. Let’s continue the exercise in this step. We want to write an if condition.
Solution (Continued)
Extending the solution is easy.
Write appropriate if, elif and else conditions.
number1 = int(input("Enter Number1: "))number2 = int(input("Enter Number2: "))print("\n\n1 - Add")print("2 - Subtract")print("3 - Divide")print("4 - Multiply")choice = int(input("Choose Operation: "))# print(number1 + number2)# print(choice)if choice==1:result = number1 + number2elif choice==2:result = number1 - number2elif choice==3:result = number1 / number2elif choice==4:result = number1 * number2else:result = "Invalid Choice"print(result)
We added the following code to account for invalid input.
else:result = "Invalid Choice"
Summary
In this step, we:
Augmented the Menu Exercise to get all the input from the console, and compute a value from them
Corrected the logic to handle incorrect input
In this step, let’s look at a few
puzzles related to these if, elif and else clauses.
Puzzle-01
Let’s start with the first puzzle. Guess the output.
k = 15if (k > 20):print(1)elif (k > 10):print(2)elif (k < 20):print(3)else:print(4)
When we run it, you can see that
the output is 2.
k has a value of 15, is it greater than 20? No! Execution goes to the
elif, is k greater then 10? Yes. It prints 2 and goes out of the complete
if-else block.
Inside the if conditional, the if, elif and else clauses are all independent
ones. Only one matching block is ever executed.
Puzzle-02
What do you think would be the output of this particular piece of code?
l = 15if (l < 20):print("l<20")if (l > 20):print("l>20")else:print("Who am I?")
Note that there are two totally
different if conditions in here :
if l < 20:
...
immediately followed byif l > 20: ... else:
....
The first if is true. l<20 is printed.
The second if is a separate statement. The
condition is false. So. else gets executed. Therefore,
"who am
I"
gets printed.
Puzzle-03
Let’s run this code.
m = 15if m>20:if m<20:print("m>20")else:print("Who am I?")
You can see that nothing is printed.
The most important thing to focus on here, is indentation.
The second if block is executed only if the
first if is true.
Puzzle-04
What would be the output?
number = 5if number < 0:number = number + 10number = number + 5print(number)
10 is printed.
The most important thing to focus on here, is indentation.
Only number = number + 10 is part of if block. It is not executed
because the condition is false.
number = number + 5 is not part of if. So, it gets
executed.
Let’s add a couple of spaces
before number = number +
5.
What would be the output?
number = 5if number < 0:number = number + 10number = number + 5print(number)
5 is printed.
Both the statements number = number + 10 and number = number + 5 are part of if block. They are not executed
because the condition is false.
Summary
In this step, we:
Looked at a few puzzles
related to if, elif and else
Explored the importance of
indentation and the different condition clauses inside an if
statement
Let’s start looking at another
important data type in Python, that’s used to represent strings. Not surprisingly, it
is in fact named str!
Let’s look at valid string representations.
>>> message = "Hello World">>> message = 'Hello World'>>> message = 'Hello World"File "<stdin>", line 1message = 'Hello World" ^SyntaxError: EOL while scanning string literal
In Python, you can use either
```or""` to delimit string
values.
type() method can be used to find type
of a variable.
>>> message = "Hello World">>> type(message)<class 'str'>
The str class provides a lot of utility
methods.
>>> message.upper()'HELLO WORLD'>>> message.lower()'hello world'>>> message = "hello"
message.capitalize() does init caps. Only first
character is changed to uppercase.
>>> "hello".capitalize()'Hello'>>> 'hello'.capitalize()'Hello'
You can also run this directly -
'hello'.capitalize(). Isn’t that
cool!
That’s because each piece of text
in python is an object of the str class, and we can directly call
methods of that class on str objects.
Now let’s shift our attention to methods, which gives us more information about the specific contents of a string.
We want to find out if this string contains numeric values?
Does it contain alphabets only?
Does it contain alpha-numeric values?
Is it lowercase?
Is it uppercase?
To find if a piece of text contains only lower case alphabets.
>>> 'hello'.islower()True>>> 'Hello'.islower()False
If the first letter is in
uppercase, then istitle() will return a True value.
>>> 'Hello'.istitle()True>>> 'hello'.istitle()False
To find if a piece of text contains only upper case alphabets.
>>> 'hello'.isupper()False>>> 'Hello'.isupper()False>>> 'HELLO'.isupper()True
isdigit() checks if a string is a numeric
value.
>>> '123'.isdigit()True>>> 'A23'.isdigit()False>>> '2 3'.isdigit()False>>> '23'.isdigit()True
isalpha() checks if a string only contains
alphabets.
>>> '23'.isalpha()False>>> '2A'.isalpha()False>>> 'ABC'.isalpha()True
isalnum() checks if a string only contains
alphabets and/or numerals.
>>> 'ABC123'.isalnum()True>>> 'ABC 123'.isalnum()False
Lastly, we look at things which you can use, to check characters of a string.
endswith is self
explanatory.
>>> 'Hello World'.endswith('World')True>>> 'Hello World'.endswith('ld')True>>> 'Hello World'.endswith('old')False>>> 'Hello World'.endswith('Wo')False
startswith is self explanatory as
well.
>>> 'Hello World'.startswith('Wo')False>>> 'Hello World'.startswith('He')True>>> 'Hello World'.startswith('Hell0')False>>> 'Hello World'.startswith('Hello')True
find method returns if a piece of
text is present in another string. Returns the first match index.
>>> 'Hello World'.find('Hello')0>>> 'Hello World'.find('ello')1
A value of -1 is returned, if you’re searching
for something which is not present in the string.
If you are searching for
'Ello' with a capital 'E' ,you’ll not be able to find it.
Search is case sensitive.
>>> 'Hello World'.find('Ello')-1>>> 'Hello World'.find('bello')-1>>> 'Hello World'.find('Ello')-1
We’ll now try and convert values from one type to another, and try and play around with them.
str converts boolean value to a text
value.
>>> str(True)'True'
All text value except for empty
string represent True. So, bool returns True for everything
except empty string.
>>> bool('True')True>>> bool('true')True>>> bool('tru')True>>> bool('false')True>>> bool('False')True>>> bool('')False
Let’s try and convert a few integer values to strings.
>>>str(123)'123'>>> str(12345)'12345'>>> str(12345.45678)'12345.45678'
Let’s do the reverse.
>>> int('45')45>>> int('45.56')ValueError: invalid literal for int()
if we do int('45.56'), you can see that it throws an
error. It says “I cannot convert this to an int, as 45.56 is an invalid
integer”.
You can also pass an additional
parameter to int indicating the numeric system -
16 for Hexa decimal, 8 for Octal etc. Default is 10 - Decimal.
>>> int('45abc',16)285372>>> int('a',16)10>>> int('b',16)11>>> int('c',16)12>>> int('f',16)15>>> int('g',16)ValueError: invalid literal for int() with base 16: 'g'
You can also convert string to float.
>>> float("34.43")34.43>>> float("34.43rer")ValueError: could not convert string to float: '34.43rer'
Summary
In this quick step, we looked at
converting different types to strings, and converting strings to different types. So
we looked at int, bool and float values, and we looked at how to
convert them to string, and how to convert strings back to these specific
types.
In this step, let’s learn an important fact about strings in Python.
String values are immutable.
What does immutability mean, and why do we say strings are immutable?
Let’s create a very simple
string: message =
'Hello', and we’re saying message.upper(). But what does it do? It prints
'HELLO', with all characters in
uppercase. Well, what would happen if you do print(message)? It says 'Hello'.
>>> message = "Hello">>> message.upper()'HELLO'>>> message'Hello'
You would see we tried change the content of message, but it has not changed.
When we execute message.upper(), a new string is created, and it
is returned back. Original string remained unchanged. This is called
immutability.
Once you define a string in Python, you’ll not be able to change the value of it.
You can use - “OK. I can do
something of this kind: message =
message.upper()”.
What would happen now?
Will the value of message get changed? It prints
'HELLO', with all
caps.
Did the value of message change? Does this prove that
strings are mutable?
The important thing you need to understand about all this stuff, is how objects are stored inside Python.
There are things called variables, and there are things called objects.
When we run message =
'Hello'
We are creating one object
of str class with a values
'Hello'.
We are creating one
variable called message
The location of
'Hello' is stored into
message
In Python, your variables are nothing but a name.
If location of 'Hello' in memory is A, then the value stored in
message is A. message is called a
reference.
What happens with message =
message.upper()?
A new object is created with
value 'HELLO' at a different location
B.
A reference ot location
B is stored into message variable.
Summary : The original value at
location A has not changed and cannot be
changed for str variables. Hence ‘str’ objects
are immutable.
Variables are just names referring to a location. They don’t really contain the value. Variables contain a reference to the location that contains the object.
One of the things that surprises people new to Python, is that there is no character data type in Python.
Typically we have text data types
in all the languages, don’t we? 'Hello
World'
for example, is text data, and we stored it in message. This is called a
string.
In other languages, you would
have something to represent a single character symbol. For example in Java, you can
have a char data type, to store a single
character ch, in which 'h' is one character. But in Python,
there is no separate data type to store single characters.
For example, let’s see how Python
treats the first character of the following string message. The way you can access the
first character of a string is by saying message[0].
>>> message = "Hello World">>> message[0]'H'>>> type(message[0])<class 'str'>>>> type(message)<class 'str'>
type(message[0]) and type(message) print the same type str. No
difference.
In Python, whether you’re talking
about a string, or you’re talking about a single character symbol, they are all
represented by the same class, str.
message[100] throws an IndexError.
>>> message[0]'H'>>> message[1]'e'>>> message[2]'l'>>> message[3]'l'>>> message[100]IndexError: string index out of range
It says: “The given index is out of the range of the value of that specific string”.
Let’s say we would want to print all the characters in this string.
The way you could do that, is by
saying: for ch in message:
print(ch).
Summary
In this short step, we looked at the fact that there is no separate character class, or data type in Python. We also looked at how do we loop over a given string, and print all the characters present inside this string.
In this step, we will introduce
you to the string module.
If we would want to use anything
from a module in Python, you need to import that specific module into your
program.
>>> import string
If you do a string. and press , it would show the
different things which are part of the string module.
>>> string.string.Formatter( string.ascii_uppercase string.octdigitsstring.Template( string.capwords( string.printablestring.ascii_letters string.digits string.punctuationstring.ascii_lowercase string.hexdigits string.whitespace
Let’s explore some of these.
>>> string.ascii_letters'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'>>> string.ascii_lowercase'abcdefghijklmnopqrstuvwxyz'>>> string.ascii_uppercase'ABCDEFGHIJKLMNOPQRSTUVWXYZ'>>> string.digits'0123456789'>>> string.hexdigits'0123456789abcdefABCDEF'>>> string.punctuation'!"#$%&\'()*+,-./:;<=>[email protected][\\]^_`{|}~'>>> string.ascii_letters'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'
You have a set of printable characters, punctuation characters and a lot more.
You can check a text value against any of these
>>> 'a' in string.ascii_lettersTrue>>> 'ab' in string.ascii_lettersTrue>>> 'abc' in string.ascii_lettersTrue
in operation on a string, checks if
a given string.
>>> '1' in '13579'True>>> '2' in '13579'False>>> '4' in '13579'False
Summary
In this step, we explored more
exercises involving the str module of
Python.
Let’s start with an Exercise - find if a specific character is a vowel or not.
>>> char = 'a'>>> vowel_string = 'aeiouAEIOU'>>> char in vowel_stringTrue>>> char = 'b'>>> char in vowel_stringFalse
he other thing you can do, is just have the capital vowels, or just the lowercase versions.
>>> vowel_string = 'AEIOU'>>> char.upper() in vowel_stringFalse>>> char = 'a'>>> char.upper() in vowel_stringTrue
Now let’s move on to the next one.
We want to find out and print all
the capital alphabets, from A to Z.
There was a small clue at the
start of the previous step, regarding importing the string module. We did the string module, and we saw that string module contained a number of
things.
>>> string.ascii_uppercase'ABCDEFGHIJKLMNOPQRSTUVWXYZ'>>> for char in string.ascii_uppercase:... print(char)...ABCDEFGHIJKLMNOPQRSTUVWXYZ
Try another easy exercise: print
all the lower characters. Instead of string.ascii_uppercase, you have string.ascii_lowercase.
>>> for char in string.ascii_lowercase:... print(char)...abcdefghijklmnopqrstuvwxyz
An even easier exercise, would be to print all the digits.
>>> for char in string.string.Formatter( string.ascii_uppercase string.octdigitsstring.Template( string.capwords( string.printablestring.ascii_letters string.digits string.punctuationstring.ascii_lowercase string.hexdigits string.whitespace>>> for char in string.digits:... print(char)...0123456789>>>
The last exercise which we want to leave you with, is to check if something is a consonant.
A consonant is an alphabet which
is not a vowel, so any alphabet which is not in 'aeiou' is a consonant. The simplest way
of doing this is to say consonant_string =
'bcdfghj...' and so on. Looks like a very
long solution? There is an easier way out.
>>> vowel_string = 'aeiou'>>> char = 'b'>>> char.isalpha() and char.lower() not in vowel_stringTrue
In the step, let’s look at a few
more puzzles and exercises related to strings. Let’s say we have a simple string,
string_example, and this is contains an English
sentence. 'This is a
great thing.'
Let’s try to to print each of the words present in this string, on a separate line.
So we would want to print
'This', 'is', 'a', 'great' and 'thing' on individual
lines.
One of the clues we’ll give you
is, try and do string_example.
<TAB>. There are a huge list of
methods, which would come up if you do that.
>>> string_example = "This is a great thing">>> string_example.string_example.capitalize( string_example.join(string_example.casefold( string_example.ljust(string_example.center( string_example.lower(string_example.count( string_example.lstrip(string_example.encode( string_example.maketrans(string_example.endswith( string_example.partition(string_example.expandtabs( string_example.replace(string_example.find( string_example.rfind(string_example.format( string_example.rindex(string_example.format_map( string_example.rjust(string_example.index( string_example.rpartition(string_example.isalnum( string_example.rsplit(string_example.isalpha( string_example.rstrip(string_example.isdecimal( string_example.split(string_example.isdigit( string_example.splitlines(string_example.isidentifier( string_example.startswith(string_example.islower( string_example.strip(string_example.isnumeric( string_example.swapcase(string_example.isprintable( string_example.title(string_example.isspace( string_example.translate(string_example.istitle( string_example.upper(string_example.isupper( string_example.zfill(
One of the methods in the list is
the split() method.
>>> string_example.split()['This', 'is', 'a', 'great', 'thing']>>> for word in string_example.split():... print(word)...Thisisagreatthing
split_lines() method looks for a '\n', and it divides the string based
on it. If you have a string which contains newlines, and you would want to divide it
into a number of strings with each line as a new element, the method you can use is
split_lines().
>>> string_example = "This\nis\n\ngreat\nthing">>> print(string_example)Thisisgreatthing>>> string_example = "This\nis\na\ngreat\nthing">>> print(string_example)Thisisagreatthing>>> string_example.splitlines()['This', 'is', 'a', 'great', 'thing']>>>
The last thing which we look at, is concatenation operator.
>>> 1 + 23>>> "1" + "2"'12'>>> "1" + 1TypeError: must be str, not int>>> "ABC" + "DEF"'ABCDEF'
In Python, you cannot do
+ operator between two different
types. + with two strings is
concatenation. + with two numbers is
addition.
One other interesting operator on
strings is multiplication. If you do a '1' * 20, What do you think will be the
output?
>>> 1 * 2020>>> '1' * 20'11111111111111111111'>>> 'A' * 10'AAAAAAAAAA'
If you multiply a string with
number, the string value is
concatenated number times.
The last thing which we look at in this step, is comparing strings.
Let’s say we have a string with a
value str =
'test', and you have another string to
with a value str1 =
'test1'.
We want to check whether both these strings are the same.
>>> str = "test">>> str2 = "test1">>> str == str2False>>> str2 = "test">>> str == str2True
You can compare strings using the
== operator.
Summary
In this step, we explored a few exercises on strings, covering areas such as:
Splitting a given sentence into individual words
The concatenation operator,
+
The string multiplication
pattern, *
The use of the == operator to compare
strings
Welcome to the section on Loops.
In this section, we will look at a variety of loops that are available in Python. We
will look mainly at the for loop, and the while loop.
Let’s start with revising the basics of the for loop, we have learned in the previous steps.
We saw that a for loop helps us to loop around the
same set of code statements, many times over.
Let’s look at a few simple examples, once again.
Snippet-01
The syntax of a for loop is very
simple.
For example, this code snippet
will tell you all about it: for i in range(1, 11):
print(i).
What does this do? Very simple,
it prints from 1 to 10.
In the call to the range() function, the second parameter
is exclusive. We are actually looping from 1 to 10, and this piece of code,
print(i), is being executed for different
values of i.
>>> for i in range(1,11):... print(i)...12345678910
for loop can also be used to loop
round the characters in a string.
>>> for ch in "Hello World":... print(ch)...HelloWorld
for loop can be used to loop around
all the words in a given sentence.
>>> for word in "Hello World".split():... print(word)...HelloWorld
for loop can be used to loop around
a specific list of values.
>>> for item in (3, 6, 9):... print(item)...369
Summary
In this step, we started with
discussing and revising basic concepts about the for loop
Welcome back to this step, where
we would do a lot of exercises with the for loop.
Exercises
The first exercise is to
find out if a number is prime. We want to write a method, is_prime(), which accepts an integer
value as parameter, and returns whether it’s a prime. (Hint: A prime number is
something which is only divisible by 1 and
itself).
5 is only divisible by
1 and 5. It is not divisible
by any other number. Same is the case with 7 and 11.
However, 6 is divisible by
1, 2, 3 and 6. So it’s not a
prime number.
The second exercise is to
write a method to calculate the sum up to a given integer, starting from
1. Hint: If I would want to find
that the sum up to 6. what’s needed is
1 + 2 + 3 + 4
+ 5 + 6.
The third exercise is to
find that the sum of divisors of a given integer. Hint: Let’s say we want to find
out the sum of the divisors of 15. The divisors of
15 are 1, 3, 5 and 15. So I would want to
calculate 1 + 3 + 5 + 15, and return that
value.
Fourth exercise is to print a numbered triangle, when given a specific integer.
Hint: Given an input 5, we would want to print the
number triangle of these kind:
11 21 2 31 2 3 41 2 3 4 5.
These are the exercises for the
for loop. We also test our skills,
with creating method and executing them, in our IDE.
Solution 1
Let’s start with creating the
is_prime() method, in a file named
for_exercises.
We would want to accept an
int parameter, and find out if it is
prime, or not.
We need to check whether it’s
divisible by any other number, other than 1 and itself. If we are passed in
a value of 5, you want to see if it’s
divisible by any of 2, 3 or 4.
def is_prime(number):
We can use a for loop. We can structure it like
this: for divisor in
range(1, number): .... We would not want to divide it
with 1, but start with 2 instead, and go up to
number-1, which is 4.
for divisor in range(2,number):
How can we check if the
number is divisible by divisor?
By using the % operator. If number is divisible by divisor we return False.
for divisor in range(2,number):if number % divisor == 0:return False
What happens if the code comes up
to the end? It would mean we tried with 2, 3 and 4, but number was not divisible by all of
them. In that case, number would be prime, and we can
safely return True.
for divisor in range(2,number):if number % divisor == 0:return Falsereturn True
For 1, the rules are a little
different, as it is neither a prime or composite. We will add an if condition to check if the number
is 1. if(number <
2):
This if condition is called a guard
check or a boundary check, to make sure that you are processing only the right input.
If number has a value less than
2, do nothing. OK, it’s not a
prime.
Here is the entire code at one place, for your reference:
def is_prime(number):if(number < 2):return Falsefor divisor in range(2,number):if number % divisor == 0:return Falsereturn Trueprint(is_prime(5));
In the previous step, we looked
at solving the is_prime() exercise. In this step, let’s
look at an implementation of sum_up_to_n(). Here is the entire code for
this exercise:
def sum_upto_n(number):sum = 0for i in range(1, number+1):sum = sum + ireturn sumprint(sum_upto_n(6))print(sum_upto_n(10))
Summary
In this step, we:
Wrote a Python function to
compute the sum of all integers, from 1, up to the input integer
n.
Let’s focus on the third
exercise, sum_of_divisors.
One of the clues we can give you,
is that sum_of_divisors() is very similar to is_prime().
You want to find out if a number
is dividing 15, and if it’s dividing
15, with the remainder of
0, then you need to add that
up.
def calculate_sum_of_divisors(number):sum = 0if(number < 2):return sumfor divisor in range(1,number+1):if number % divisor == 0:sum = sum + divisorreturn sumprint(calculate_sum_of_divisors(6))print(calculate_sum_of_divisors(15))
In this step, Let’s look at the
last exercise - print_a_number_triangle.
For example, if we call such a
function with input 5, the output needs to
be:
11 21 2 31 2 3 41 2 3 4 5
Let start with a simple thing.
Let’s try and print 1 2 3 4 5 first, and then we would look at
how to print the rest of the output. Lets proceed with defining this
method.
We can say def print_a_number_triangle(number):
... that
takes a number as an input. You want to print a sequence of integers starting from
1, up to that specific
number. How can you do that? Let’s try
this: for i in
range(1,number+1): print(i) What would happen? Let’s call
print_a_number_triangle(5) now. It
prints:
12345
on individual lines.
To print this sequence on a
single line, let’s delimit them with <SPACE> instead. Call print() like this instead: for i in range(1,number+1): print(i,
end=" ").
Let’s see what would happen now.
1 2 3 4
5
To solve our exercise, we want to repeat this again and again.
Yes, we need another for loop around it!
for j in range(1, number+1):for i in range(1, number + 1):print(i, end=" ")
Make sure that you have the
indentation right. This is called loop within a loop.
The output of above program is
1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5
1 2 3 4 5
Let’s add print("\n"), so we have a new line at the
end of each outer loop iteration.
for j in range(1, number+1):for i in range(1, number + 1):print(i, end=" ")print("\n")
Output
1 2 3 4 51 2 3 4 51 2 3 4 51 2 3 4 51 2 3 4 5
We are printing a square, not a triangle.
What we want to do is to print up
to 1 in first line, upto 2 in second line and so
on.
How can we do that? Think about it.
When you are inside this loop,
you can see the variable j.
Instead of number+1, let’s say j + 1.
When j has a value of 1, for will print from 1 to 1. When j has a value of 2, print from 1 to 2, literally printing
1
2. When j
has a value of 3, I’ll print from 1 to 3. Let’s try this and see what
would happen.
for j in range(1, number+1):for i in range(1, j + 1):print(i, end=" ")print("\n")
You can see that our number triangle is ready!
11 21 2 31 2 3 41 2 3 4 5
Here is the entire code for you:
def print_a_number_triangle(number):for j in range(1, number + 1):for i in range(1, j + 1):print(i, end=' ')print()print_a_number_triangle(6)
An important point to note is, a couple of these things can be done in a much simpler way. We will look at these options when we talk about functional programming.
Summary
In this step, we:
Presented a solution to the exercise for printing a number triangle.
Let’s look at one of the other
loops which is present in Python, called the while loop.
In the for loop, we can specify the range
of our iteration, by using the range() function.
In a while loop, we specify a logical
condition. While the condition is true, loop continues running.
Do you remember one place where
we use the condition until now? It was in an if statement.
Let’s see how to use a simple
while loop.
Snippet-01:
>>> i = 5>>> if i == 5:... print("i is 5")...i is 5
Let’s say i has a value of 0, and we then do: while i < 5:
print(i).
>>> i = 0>>> while i < 5:... print(i)...00000000^CTraceback (most recent call last):File "<stdin>", line 2, in <module>KeyboardInterrupt>>>KeyboardInterrupt
If we leave it to run, you’d see
that it continuously prints 0 again, and again. Let’s do a
<CTRL-C> or <COMMAND-C> to interrupt
this.
What is happening here?
Initially i is 0, and the condition i < 5 is True, and print(i) is executed. Next iteration, it
checks the condition, it is True, and 0 is printed. This continues to
happen. What’s happening is an infinite loop.
One of the important things to
make sure in a while loop, is to increment the value
of i. We need to say something like
i = i +
1.
>>> while i < 5:... print(i)... i = i + 1...01234
So how does it work?
*i initially had a value of
0. First the condition is checked.
It’s True, so 0 is printed and then the value of
i is incremented to 1.
i is still less than
5, so the loop continues to
execute, and this happens until 4 is printed. i again gets incremented to
4 +
1, or
5.
Then we check the condition
i <
5.
This is now False. Control goes out of the
while loop, and terminates
it.
When executing a while, control flow is just based on a
condition. As long as the condition is True, we keep executing the code. An
important thing to remember, is to make sure the control variable is
updated.
>>> for i in range(0,5): print(i)...01234
A for loop is much simpler to code
than a while. With while, we have to write an expression
statement, to increment the value.
The question you might have is - What are the situations when you should use a while?
We will look at that very soon.
Summary
In this video, we:
Were introduced to the
concept of a while loop in
Python
Understood the importance of a control variable being incremented inside the loop
Observed differences
between the working of a while, and a for loop
In the previous step, we were
introduced to while loop. In this step, let’s look
at a couple of exercises using the while loop.
Exercises
print_squares_upto_limit(30): We need to print all the
squares of numbers, up to a limit of 30. The output needs to be
1 4 9 16
25.
print_cubes_upto_limit(30): We need to print all the
cubes of numbers, up to a limit of 30.The output needs to be 1 8
27.
Exercise 1: Solution
Here is the entire code for your reference:
def print_squares_upto_limit(limit):i = 1while i * i < limit:print(i*i, end = " ")i = i + 1
Now the next exercise, was to print cubes up to a limit.
The expression in the
while condition should now be
i*i*i <
30.
def print_cubes_upto_limit(limit):i = 1while i * i * i < limit:print(i*i*i, end = " ")i = i + 1print_cubes_upto_limit(80)
Could we have implemented above
two examples with for loop? It would’ve been a little
more difficult.
Typically, we use a for loop when we know how many times
the loop will be executed is clear at the start.
If we do not know, how many times
a loop will run, while is a better
option.
Earlier we used if statement to implement a
solution for this:
Ask the User for input:
Enter two numbers
Choose the Option:
1 - Add
2 - Subtract
3 - Multiply
4 - Divide
Perform the Operation
Publish the Result
We would want to enhance it to execute in a loop multiple times, until the user chooses to exit. We will add an option 5 - Exit.
Ask the User for input:
Enter two numbers
Choose the Option:
1 - Add
2 - Subtract
3 - Multiply
4 - Divide
5 - Exit
Perform the Operation
Publish the Result
Repeat until Option 5 is chosen.
Snippet-01 Explained
Here’s the earlier code we wrote with if:
number1 = int(input("Enter Number1: "))number2 = int(input("Enter Number2: "))print("\n\n1 - Add")print("2 - Subtract")print("3 - Divide")print("4 - Multiply")choice = int(input("Choose Operation: "))# print(number1 + number2)# print(choice)if choice==1:result = number1 + number2elif choice==2:result = number1 - number2elif choice==3:result = number1 / number2elif choice==4:result = number1 * number2else:result = "Invalid Choice"print(result)